Aluminum OR brass FOR COOLING?

You two arent basing your temp reading onff the stock gage, are you? I hope not.

Assuming your 3-core brass radiator is in good shape (fins not damaged, no deposits on the inside) it should be adequate for all but the most demanding engines.
There are several other variables that influence coolant temps. such as:
1. Pump function/condition
2. Coolant mix (%distilled water, %coolant, additives like Water Wetter, etc.)
3. Thermostat function/condition
4. Air/fuel ratio (lean=increased temp)
5. Fan cfm
6. Access to cool air
7. Underhood temp.

Agreed in all respects. I just recently went to a griffin 31x19 2 row aluminum radiator and it cools like a mother in my gt (in other words with respect to the stock gauge, it never really make its it to the 3rd tick mark on the gauge). Even on these 90-100 degree days the only time my fan cuts on (set to switch on at 200) is if i'm sitting at an idle for a while. Even when the fan cuts on it will cut back off after about 20 seconds because it cools the car down so well. Other than that it's never on. Almost scares me thinking about winter time however, it just cools so well.

Aluminum does not cool any better than copper it's more for getting weight off the front end. A 3-row copper raditator is pretty heavy though. My car runs really cool.

An when an aluminum is only $30 more than a brass, its no big deal if they cool the same. Brass heat exhangers can work very well I agree. I had a "high efficiency" brass 3-row in my '65 Mustang and it cooled at least as well as a regular 4-row. You could see the t-stat cycling on the gage.

30 dollars more for an OK aluminum radiator. If going with an aluminum radiator you want the top of the line. Aluminum is hard to weld and I heard some of the cheaper ones are prone to leaking.

I think the thermostat had alot to do with cool how car runs.

check your iat(intake air temp) sensor, my car was running hot and was pinging over 3k. when i pulled it out the iat it was covered in oil(running a k&n)and grime, changed it and all my problems went away right then.

Lots of stuff to play with here - not sure where to start. I doubt anyone has a brass radiator. Older common construction of radiators is COPPER fin-tubes (commonly called the coil or core) and BRASS end tanks. (edit - some VERY old - antique - cores were brass) So most of you talking about brass radiators probably have copper cores. Over time, the efficiency of any radiator drops. Debris clogs the fins which limits air flow and thus heat transfer. Internally, deposits form on the inside of the fin tubes, and they also inhibit heat transfer. The designers build in extra capacity so that as this degradation in cooling potential occurs, the radiator can continue to do it's job for a good long time. Even if you're diligent about keeping things as clean as you can inside and out, most 10-15 year old Stangs with more than 50K on them, with stock radiators are likely approaching the point where the radiator just won't keep the car cool anymore because of the degradation described above. So almost any new (properly sized) radiator is gonna perform WAY better than the tired one - regardless of size, material, etc, provided of course that everything else is up to par in the cooling system.

Contrary to popular belief, copper has better heat transfer characteristics than aluminum. As years have gone by, manufacturers have switched to aluminum cores with plastic end tanks primarily because of their lighter weight and lower cost. So, all else being equal - if the most heat transfer is your goal, and you don't mind the weight, a copper core provides more heat transfer capacity than an aluminum core of the same size, number of fin-tubes, and equal number of fins/inch of tube.

1) Get an accurate gauge that will give you more accurate indications of your running temps. The stock gauges are marginal at best.
2) A stock replacement radiator has plenty of capacity to keep even a modded 5.0L running plenty cool - provided the rest of the cooling system is in good working order. AutoXr1 did a great job of hitting on most of those system components that need to be considered.
3) If you feel you need extra cooling capacity in the radiator, the following things should be considered:
a) Copper cores transfer more heat than aluminum cores, but weigh more - you have to choose.
b) All else equal, extra length and width in the core will net you more additional capacity than adding additional rows of fin tubes will. Example - increasing from a 20"X20" size (400 sq. in) to 20"X30" (600 sq. in) will net you 50% more area, and close to 50% more heat transfer capacity. Increasing from 2 rows of fin tubes in the core to 3 rows of fin tubes (50% more fin tubes) will only get you about a 12-15% increase in capacity. The reason is that as the air passes over the first two fin tubes, it takes heat from them which heats the air. By the time it gets to the third, or fourth rows, it's much hotter, and capable of removing less heat from the coolant in those fin tubes.
c) The most commonly overlooked part of picking a radiator or core is the number of fins per linear inch of fin-tube. Contemporary cores (aluminum or copper) can be manufactured with a high number of fins per inch of fin tube. If you look at older radiators, it's not uncommon to find something on the order of 8-12 fins per inch. Newer cores can come with as many as 16-18 fins per inch. All else equal, heat transfer capacity increase is directly proportional to an increase in the number of fins per inch. And of course, there's a limit - at some point too much fin density interferes with air's ability to easily pass through the coil. The good news is you won't find anyone making fin-tubes that are too dense to allow good air flow. It's increased fin tube density that usually makes a new aluminum radiator of the same size and rows of fin tubes perform so much better than an older copper core of similar size. Usually the newer aluminum unit has more fins per inch than the older copper unit. Without knowing that, many people simply conclude that aluminum has better heat transfer characteristics than copper. Not true.
4) None of this means anything if you don't allow necessary airflow across the coil. So be sure your fan is doing it's job, and that at speed air can flow through the coil. It sounds elementary, but at speed for air to move across the radiator, it must be able to exit the back of the coil - in other words, it's got to be able to leave the engine compartment. Most contempory cars have some sort of small air dam under the car underneath and in front of the radiator, plastic lip, or lower engine compartment cover. These devices are almost always there to create an area of low pressure behind and below the radiator, so that at speed the air that passes over the coil can easily exit the engine compartment. Some people don't like the way they look, or they mangle them on curbs. When damaged or removed, many cars will have cooling issues at speed - because the air passing over the radiator simply can't leave the engine compartment quickly enough - which limits how much air is passing over the radiator.

I had a contemporary 2-row copper core (18 fins/inch) installed in the stock Volvo diesel radiator shell/brass tanks. It's only 1.5" thick, and is 24"x16". With a/c in the summer/south, it has more than enough capacity to keep the motor cool (about 270HP/340 torque). The minute the fans come on, or I start moving above 30-35 mph, it cools right off. And in the winter time, the fans barely operate at all. The downside is that it's heavy. But, a custom aluminum radiator (nobody makes an off the shelf for my application) is expensive. Mine cost $180 at the local radiator shop (including several custom fittngs for switches, drain cocks, changing hose locations, etc.), and if anything ever happens to it - its simple soldered copper/brass construction will allow any radiator shop anywhere to be able to repair it.

I obviously have time on my hands this morning.