63 Dart 383

Exhaust clearance issues, potential firewall/transmission clearance issues, potential cooling system upgrades, stronger torsion bars, shorter driveshaft, etc. I'm sure there's probably more.

If you're having trouble figuring out the 318, I'd stick with a smallblock.
Coolant system and possible torsion bars will come with the donor. There's a mount kit that moves the engine forward for the clearance issues. That may clear up the drive shaft issues as well. I'd rather go with an early smallblock, but this one is available and cheap.