engine swap

[84LX89GT]

I believe the 3.8 and 5.0L in stangs had the same crossmember but different mounts (of course), also you would need the fuel sending unit out of the carbed car (IF it would fit), you could also have an external fuel pressure regulator before the carburetor that uses the stock fuel pump/line system (my '85 GT has a HUGE electric pump and an external regulator for it to drop fuel pressure down for the carb).
You'll need probably a stand-alone MSD ignition system (distributor, controller) I have a MSD system on mine and the distributor has only mechanical advance.....cool because you can tune it for total timing and advance curve very easily and there's hardly any wires.....real simple.
Exhaust system should be easy - 5.0 system for whatever model year vehicle you have (you didn't mention), you may have to relocate brake lines in back or just have dumps before the rear axle.

You'll need a radiator for it as well, an 8.8" rear axle if you plan on using any kind of power.

If you're going to put a windsor engine in, i'd recommend just putting in a 351W since you're going to have to buy all sorts of stuff anyway. If you're doing the junkyard/budget way, a 5.0L an be put in real cheap....or buying a 5.0L junker and swapping is also a good idea.