I wouldn't use the CFI system as is, since the throttle body that came on them is horribly restrictive, and the rest of the system isn't the best. It's a tough call, but I'd probably go carbureted before I'd run early Ford CFI (and I'm a fuel injection nut!).
Your better bet would probably be to hunt down an 87 or later engine, 302 or 300 as you prefer. I have a 302, but would go for the 300 since it's honestly a better truck engine.
Your other good alternative would be to snag a later set of multipoint EFI intake manifolds for your 302, but since the 84 had lousy heads (with very different flow characteristics) and a different cam, a factory truck speed density system won't run it worth anything. Best bet would be a mass air swap and running it off a Mustang computer, which can be done cheaply if you know where to scrounge the wiring harness and where to get a workable computer cheap (topics the board should be able to help ya a bunch on). We're doing something very similar to an 85 F-250 351W in the near term (once my cab swap is done), and have gone through the same selective scrounging process. :thumbup If you still have access to that 86 EFI 302, you can get a lot of your parts from it, including the manifolds, most of the wiring, and the fuel system.
If you do want to swap in the 302, you'll need the appropriate 50-oz flywheel or flexplate (do you have a manual or automatic?). Motor mounts themselves are different, but if I'm not mistaken I believe the six and the small blocks both use the same frame brackets -- search should clear that one up for you.