TH350 in a 4WD environment behind a big block is a nice recipe for failure...3rd / reverse pack failure is very common...
Time for a TH400 to be put in where the 350 currently resides...
TH400 clutch packs should be maxed out with thin steels and Red plates...HD band...and a good shift kit...make sure vacuum modulator is correctly plumbed and ported to full engine vacuum, not timed vacuum on carb.
You will need to put in an electric kick down switch system...
Most important is an aftermarket trans oil cooler that bypasses the engine radiator totally...
Have fun with Winter mate...
