Hello,
I agree with the above, would be more cost effective to have the TH350 built
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To swap to a TH400, you'd also need to wire an electric kickdown switch. Original switch was part of the gas pedal, but some aftermarket installs use a carb-mounted switch.
Being a '78, I'm assuming you have a 203 transfer case now. As mentioned, the adapter and input shaft on the case would need to be changed and they are RARE. You'd also need the correct output shaft in the transmission, as the 203 takes the shortest of all TH400 tail shafts (also extremely rare). Advance adapters may have a solution, not sure...
Either way, the TH350 can easily be built to take big block torque. Heck, my Dad ran one behind a 454 for many years in his K30 T-tag. That weighs 6,800 lbs and the transmission held up for nearly 20 years
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Hope this helps,
Joe
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