I used to get this problem on my '81 after doing around 90-100mph for a while. The temp would climb to a lot hotter than normal, but would drop back down once I slowed down a bit. This was both before & after the rad had been re-cored. Fan clutch was new & water pump was OK, although the seals around the rad had seen better days.
What I found was that fitting a tranny cooler has caused my coolant temps to drop way down &, since fitting it, I have never seen it go over 200. I've got a 180 stat & the coolant temp sits at about 180 & hit 190 after some "spirited" driving in ambient temps of 100F (what a fun day that was
). People don't seem to believe me about the tranny cooler but, remembering my college physics, the rate of cooling depends on various things, one being the temp differential. Normally the coolant is pushed through the rad, where the cooler air passing through takes the heat away. But, with the stock system there's also very hot transmission fluid being circulated through the rad. This fluid can be very hot when it goes to the rad, so it's robbing the coolant of some of the cooling benefits of the airstream & could also be heating it up!! So, even with a nice stream of cool air blasting through the rad, there's still very hot tranny fluid going into one end of it, which just ain't gonna help the coolant lose heat. If you've got a manual then none of this applies, but if you've got an auto then you'll probably find that a SMALL tranny cooler fitted to cool the fluid before it goes into the rad will help a lot. It'll also give your tranny a longer life, but I found that a small cooler was all it needed to get the ideal running temp. A large cooler will probably cool it too much for normal street use.
Another thing with cooling is that you want to push the fluid through the rad quite quickly. As cooling depends on the temperature differential, then allowing coolant to meander through the rad won't be very efficient as it will cool down while in there, decreasing the temperature differential & therefore the amount of heat transfered. You need to push the hot fluid through quickly, so that more heat is transfered to the airstream.
Thermostat value will only affect the minimum normal operating temp. If the coolant is, say, at 220F then a 160, 180 or 195 stat ain't gonna have the slightest bit of difference as all 3 will be wide open.
I'd be very interested in hearing if people with manual transmissions have many overheating problems (assuming the cooling system hasn't got any major problems). Fitting a temp guage in my tranny pan has shown me that tranny fluid can reach very high temps, especially in low gears, or at high speeds, & this hot fluid is just going to compound any cooling problems with the system.
Lastly, since fitting the tranny cooler, I've fitted an aluminum water pump (stock flow rate) & REMOVED most of the visible radiator seals (they were perished and looked horrible, it was an aesthetic thing
) & I've still got no overheating problems (ambients were in the 90's).
