Just something to think about with tranny coolers: I have never used one in conjunction with the radiator trans cooling lines on any of my cars that I have used them over the last 30 years. My first application was in in the very late 70's on a Nova SS with a Turbo 350 like in your C3 running 2 coolers in series outside of the radiator. The Nova and your 78 are ideal for a cooler outside of the radiator since they both have a mechanical fan that runs all the time, always pulling cool air over the coolers. 2 things kill automatic transmissions: heat and dirt (not changing the fluid often and the filter). I beat the crap out of the Nova trans and when I sold the car the OEM 350 Turbo was still shifting great at 170,000 miles. Rather than have the radiator heat the fluid or try to cool really hot trans fluid, let the trans cooler do all the cooling like I did and do today on 2 of my cars with electric fans. I would put the biggest double stack cooler you can fit in the grille and let it do all the cooling.
Due to space limitations, I currently run 2 Long double stack coolers (do not use Tube and fin coolers which are MUCH less efficient) in my 94 Mustang GT convertible (total GVW rating of 42,000 GVW) and 2 Long Double stack coolers in a 2001 Pontiac Grand Prix with a total rating of 36,000 GVW outside of the radiator cooling and have had no issues now for 10 years. The GP has 135,000 miles on the junk 4T65E FWD transmissions which are not very durable. I can tell you that the trans fluid gets REALLY hot in normal driving. Even though the electric fans only operate when the car is idle for a bit and when the AC is on, I have not have any issues and check the fluid temp with an infrared thermometer occasionally-I have never seen temps over 160 degrees in the Mustang GT and never over 145-150 degrees in the GP in the hottest weather). I run the GP all year round even in 0 degree weather in New England and my only issue is that the trans takes about 2-3 miles of driving in those temperatures before it will shift into overdrive (trans fluid has to be at 88 degrees for 4th to engage). i just take it easy which you should do anyway in very cold temps. Hope that helps!
Last edited by jb78L-82; 05-06-2013 at 08:28 AM.