If you have an 80's 302/5.0 already, you have all the parts to swap in another late model 5.0 engine. You just need to swap over the accessory drive that you have and works and whatever flywheel/flexplate you are running. That's the stuff that trips most people up.
Since it sounds like you don't know what engine you have I wouldn't bother swapping EFI onto it. While EFI is very adaptable, why not just get the engine that the EFI was matched to? Get the whole package and not worry about if you have the correct distributor gear, if you need to swap the firing order on the injectors, what size injectors should be run, etc.
As for which EFI, simple, Factory Ford EEC port injected. Which one of those engines gets to be a toss up. The mustang EFI has been a steady go to choice for decades. Although lately the Explorer EEC-V has come to its own. And that engine gets a lot better intake, and the greatest front accessory drive. But headers can be a pain if you get the P-heads.
