Rear end center section - how difficult to swap out?

You'll have to put the car up on jackstands and remove the wheels. You'll have to pull the axles, so if you have a drum rear, you'll pull the drums off, then loosen the 4 nuts near the axles that also hold the backing plate on by turning the axle so the hole in it allows access to the nuts with a socket. You should be able to pull the axles out enough without having to remove the brake lines to the wheel cylinders or the emergency brake lines. At worst, you'd have to remove the brake lines and rebleed the brakes, but I think you can do it without removing them. Remove the ubolts that hold the rear u-joint in place and drop the rear of the shaft without pulling it out of the transmission. You can wrap some masking tape around the end of the u-joint so the caps won't fall off if you'd like. Good time to pull the caps and inspect them anyway. Now you can remove the nuts holding the carrier in the housing. If the carrier has never been removed, there will be some brass washers that sort of dig into the threads on the studs. A small flat bladed screwdriver or a small chisel to cut them will help to remove them. You'll need to put a pan under the third member or you'll end up with smelly rear end grease all over the floor. Once you crack the bottom open, let it sit so all the grease can run out. If you have a problem pulling it out, an axle still may not be pulled out enough. Then it's a matter of cleaning up a bit and reversing the process.

Also make sure you have a fluid filling plug, either on the center section or the housing. All is not lost if you don't have one, either drill one or fill the rear through one of the axle shafts prior to installing the axle.