Engine runs hot and misses at idle.

The radiator puked out the overflow after I had shut the car off. I had it idling for some time to see if I could chase down a miss it has while idling. The car runs fine when the temp is 200F or below. Above that it has a slight miss at idle and it has a flat spot right off idle. I've cleaned the carb and rebuilt it. I've played w/ the timing, adjusted the vac advance. The ignition is a Pertronix Ignigtor II with an MDS Blaster 2 coil. I've plugged the PCV to see if that's the problem but to no avail. The car pulls fine from part throttle to full throttle all the way 'till "redline".

The engine is out of a '64 Falcon I built back in 91 and approximately 6000 miles.
I never had a problem withe the idle or the engine running hot in the Falcon. When I had the Falcon I was stationed in Virginia so the climate may have something to do with it but when I put the engine in the Mustang I also added a direct fit aluminum radiator w/ an electric fan to help compensate for the hotter climate in Texas (where I am now).
I must be missing something, I can't figure out why it would get hot like it does in the Mustang and not the Falcon. The other difference is The Falcon had a 2 1/4 exhaust with Dynomax turbo muffles couple to a set of Heddman Headers. The Mustang has Tri-Y headers and a 2in exhaust with generic muffler shop turbo mufflers. Could it be the exhaust is too small and not allowing the engine to get rid of the heat fast enough? I know this was a problem on the Ironheard XR750s back in the seventies but never heard of the problem on an automobile. The car has set for several years as well and I am trying to work all the bugs out of it. I'm stumped. Does anyone have any insight?

A few more things...

I forgot to mention the off idle stumble was really bad until I realized the vac. advance was not connected. After I connected it to ported vacuum it doesn't stumble as bad.

I never set the timing by a light I reckoned I'd do it by ear, however w/o a light I have no base line. I had it where it would detonate at heavy acceleration at part throttle but not full throttle which is fine. I backed the timing off a bit and had some minor detonation. Still gets hot though.

Thanks,
I'll check that out, the vacuum line connected to the distributor does fit a little loose. And I definitely put a timing light on it to see where it is. I may be way to advanced. I hope it is that simple.

Update to problem

Well I put a light on it and discovered I have no indicator on the timing cover. I guestimated where it was then marked it and the pulley where it is supposed to be. I was close and started to reard the timing little by little 'til it wouldn't ping anymore. I also replaced the vacuum lines. The temp guage (aftermarket mounted under the dash) was bad. So I connected the original factory guage and stuck a dial thermometer in the radiator wher the water flows in from the engine. The needle on the guage gets no higher than a little past half way and the thermometer stays between 180 and 210. When idling for long periods of time the thermometer gets about 215 to 220 but the radiator doesn't puke. The idling as smothe out and does have a slight miss only at idle eventhough I have tweaked it many times. I'm wondering if it could be the plug wires, but it doesn't miss above that.