long response, practical advice; been there, done that
Your stock cooling system should handle your 306 no problem IMHO. Here is what I started with when I bought my car and how it ended. FRPP aluminmum rad, 191 Ford stat, underdrive pulleys, bra (important piece of info here!) new hoses, ran way cool on the guage, two notches up, which made no sense since the overflow bottle would fill and under the hood was really hot. Step one, replaced the temp sending unit, "burped" the cooling system, guage read right and engine was really heating up in traffic, 220 or so and crept up the more you idled it. I agree, a quality after mkt guage is even better for checking things out. two, figure out if (you have already) you have an air flow across the rad problem (means you more than likely you have a "clean" cooling system), three start with the obvious. Any obstructions to the front openings (remember the bra? was covering up about 1/3 of the total area), make sure your air dam is in good shape, rad cap holds pressure (local auto store has tester and will do for free) and Tstat opens when it is supposed to (boiling water test), is your timing too advanced or too retarded, four are you running the shroud? (mandatory!) and if you want to put a new rad in and keep the original fan set up make sure that with the rad installed it fits right where the stock one did. The FRPP rad does not fit right at all ("slight modification required"according to FRPP catalog my aching you know what!) and I yanked it out and put in a 3 core, stock dimensions, which I explain later. If the core is thicker than the factory rad (the FRPP is) the air dam isn't as effective and the shroud will cover the fan blade tips completely (more on this later). You can buy a three core direct replacement for your two core. Its dimensions are exactly the same as stock. five, is the fan clutch shot (usual signs are dirt and gunk around where it fits into the fan on the rad side, that slick silicone feel and black color) forget the "turn it" resistance test, it doesn't tell you anything; there is a police/h/d option clutch, search for it and do not get a cheap import POS if you suspect your clutch. Is the fan cracked?(new design by Ford works really well but double what a cheap knock off runs, get from Ford dealer is the safest bet, (Ebay is dicey hard to tell what you are getting), six, do your fan blade tips stick out of the lower half of the shroud about an eigth of an inch or more? They must for the fan to be effective, that's why the fan tips are so funky looking! Also, if you are running underdrive pulleys (pulled mine because I hated the way the alternator was always undercharging the battery and the obvious water pump speed at idle) all bets are off depending on where you live. My engine now lives at exactly 195 degrees, idles forever with the A/C on and life is good.
Ford put a lot of engineering into the design and it works really well if all of the parts are working. It was designed for little old ladies to leave the A/C running at full blast at idle while they go shopping in Tucson at the mall for an hour in the middle of August. Sorry for the length of the post, but a lot of guys put in 400 dollar radiators and 350 dollar electric fans when just making sure the stock set up is working as designed will cure your ills.
