I'm replacing my FMX with a built C4. There are 2 coolant fittings on the trans that will run to the cooler, one at the front of the trans and one at the rear. Which is the pressure line from the pump? I'm running to a B&M plate type cooler. I want to make sure the lines are connected properly. Any advice?

 

The front line is pressure, rear is return.

Why are you replacing an FMX with a C4?

 

Your profile says F100, but a 65 F100 would not have an FMX transmission. It should have a Cruise-O-Matic (MX) whether you have a 352 or a six cylinder, and that's a better trans than a C4. You won't see a C-4 in the F series till 1967, and a FMX wasn't even available till 1969, as it was developed for the new 351 Windsor engines, and used in cars only.

No FMX's were used in F Series trucks till 1973, and then only on the 6 cylinders, and a few 302's.

 

The engine in the truck is a 289 from a Mustang. I'm going with a C4 mostly for price and performance. A C6 sucks up too much power and an AOD is too expensive.