JT is 100% correct (hissin50).
The stock radiator must have been designed for the 4 cylinder models. In fact, most four cylinder cars come with better radiators than what's on a stock foxbody. Ford from 79 - 93 upraded everything but the radiator. Plus, take in consideration that crap has probally built up in the radiator over the years and you can't get all of it off, maby some of the loose stuff, but the hard, baked on stuff won't come off.
You can get the AFCO or Griffin radiator for $180-190. They arn't cheap or anything, they just don't come with the tranny cooler, which should be run with an aftermarket external cooler anyways. A 3 core copper/brass goes for about the same amount, but isn't won't quite do as well, go alumimum.
I am glad JT has some experince with those high-flow T stats, I have been wanting to try one.
Also, if they do go bad, they get stuck in the open position, a definate plus.
Check the fan clutch, when the car is warm it should actually be blowing air. You should feel resistance (shut engine off first) when you try to spin the fan by hand. If not, consider a replacment. Either a good electrical or a setup like JT has, metal fan with heavy duty clutch.
I would go ahead and order a AFCO or Griffin radiator and a 180* Mr. Gasket T-stat. While your waiting for it to arrive, drain all the coolant out and add water and the Prestone Superflush and run that for a few days. Then when you install the new radiator, leave the t-stat and upper hose off and keep the garden hose in the radiator and run the engine with so it will flush it all out. Then drain everything, install the t-stat and upper hose, add about 15-30% antifreeze, a bottle or waterwetter or two, and the rest with distilled water.