having owned a 400 and the 350 transmissions I would not spend or do all that work for the 400 to fit.
I had a 350 tranny in a gm sedan for 17yrs and 380K miles . the only part I had to replace was the tail shaft bushing. I would install a 10K lbs tranny cooler with the 350 tranny. change the fluid every 20-40K miles and use dexron VI not dexron III.
anytime a transmission blows up all the tranny lines and radiator fluid system must be cleaned very completely with a flush solution.