I'm typically a small block guy, but given the two engines that you're choosing between I'd say go with the 383, sounds like a much better known quantity.
If you're running a 727 in the car now you shouldn't have to do any work to the driveshaft, but if you've got a 904 in the car now you'll have to change the driveshaft length too. I would upgrade the torsion bars at some point, and add sway bars if you don't already have them. But that's not something that you'd have to do at the same time as the swap.