The bellhousing and torque converter is pretty much a bolt-in swap. You'll probably have to fabricate a driveshaft, and possibly a rear crossmember. Given the choice, I'd pick a TH 350, it's lighter weight than the 400. If you want overdrive, try an early-model TH700, before they went to computer control on the converter clutch lockup. The early ones had a vacuum switch and a single wire going to a pressure switch, and the clutch locked in by governor pressure. Later models used the vehicle's engine control computer and the Vehicle Speed Sensor in the speedometer head to control TCC lockup. Lots of wiring headaches with the late-model oners!
Jerry

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