78 chevy with 350

stempak:
T400 a shift kit and tranny cooler you should be good to go. The less the tranny slips with a shift kit the less the heat the less the burning of the clutches. Big cams and auto trannies don't mix well, You might also have to lossen up your converter.

Or you build your 350 if that's what you have

Shift kits are greatly overrated. They are more about ego than true need. Old trannies like THM400 are always "slipping" because they do not have locking converters and because if this there is always about a 6 to 8% slippage in them and extra fuel usage. Loosen up torque converter is a bad idea off road because it forces tranny to ride stall more and stall makes heat. Basically most tranny failures in 4x4 are because they have been cooked. Many a 700R4 has gone to a early grave because people have lifted their 4x4's and not regeared and cook tranny in OD and blamed tranny and rather than regear 4x4 they retro to a older tranny with reduced potential and a much taller first gear too. THM400 was a good tranny 25 years ago but today there is better options.