If you want to run a mechanical fuel pump, you need the eccentric parts from a carb'd small block, and the timing cover from a carb'd small block. Not sure without checking years, but you may also need to change the cam retainer plate to clear the eccentric. They were different thicknesses depending on which eccentric and probably different thickness for the EFI engines. I'd say use the cam plate off the same engine you get the eccentric from, that way you know it's right.
Edit:
I'd bet you already know this, but just incase you didn't...
You need a front sump oil pan and pick-up, as well as making sure you use the correct imbalance weight for the engine you choose as a donor. If you use a roller cammed engine, they generally use a different distributor to cam gear.
If you use an Explorer engine with "P" heads, you must be careful on which headers you choose. Many are not compatible with the spark plug location unless you are prepared to do some custom mods. The Explorers have EFI set ups on them that are worth some money if you don't use them. I paid 75 bux for the entire EFI off an Explorer engine, which is pretty much normal, but I have seen people pay more. If you get at least that out of the intake and related parts, that will help recoup expenses on your project.
If you find a good running 86-92 HO engine (Mustang, Mark 7, and some T-birds), they have forged pistons from the factory. Starting in 93, the pistons went hyperutectic, which have some benefits, but are not as strong.
If the engine needs building and overbore, then the forged pistons don't mean any benefit at all.
Remember that not all EFI 5.0 engines are HOs!
Mustangs/Mark 7s are the sure bets, beyond that, many look the same, but are not, and there are differences in internal engine parts. No trucks are HO spec. If no one has swapped intake plates, HOs will say "5.0 HO", while passenger engines will say something more like "5.0 EFI".
We bought a low miles, but rusty, Mark 7 (HO and AOD) recently for 500 bux. Pulled the drivetrain and it functions perfectly. (We converted from carb to EFI though.)