Don't use the stock Caddy flex plate with the cutouts, if you're going to be making some serious power...
When doing a swap, make sure your driveshaft isn't too long. This can wipe out the thrust bearing and crank . . .
The two groove AC, P.S. belt system is too close to the HEI Distributor… Use the single belt system.
Before starting a fresh motor, prime the oiling system by spinning the oil pump drive 'til you get pressure. It's easy and if something's going to leak, it'll save a big mess.
The TH400 and 4L80E will handle the torque... Anything smaller will be on borrowed time.
For starters go "easy" on the gear ratio to start with on your Caddy. These are not "mouse motors".
Use a real fan (OEM is best) and a good fan shroud whenever possible (far superior to electrics).
Try to retain that little strap that supports the end of the starter to the block. Without the strap, if your engine kicks back against the starter, it'll be swinging in the breeze by its wires. Repalcement straps are available.