The differences are mostly preference, and simplicity. There are some gains with EFI, if your goal is maximum power (easier to get with programming). An optimally configured MFI can match that, but requires significantly greater planning. Mileage can be equal between them, but again, it depends on planning and configuration. For the purpose of simplicity and reliability, MFI is usually preferred, and what the military (HMMWV) requires. A mechanical system, plainly, has fewer things to go wrong, and is more simple to make repairs when something does go wrong.

To start with:

Source a 4911 IP, or use the pump currently on your 6.2L (turn it up a bunch, though). The 6.2L pump will work fine, but it probably won't last as long, for a number of reasons. You'll also need a set of injector lines for your pump/injector combination. Start shopping for a good 4911 pump (the pump used for 92+ MFI).

Install a mechanical wastegate control. Turbomaster is OK.

1985+ glow plug controller.

Correct throttle cable (meaning, you don't have to install the electronic APP from the 97). The original in the 1991 may fit, I don't know.

Use the ignition coil wire to power the IP. Install 6.2L temp senders for HPCA (cold start timing advance) and GP control.

Install a relay for lift pump operation (using the OPS to relay). Or, install a mechanical fuel pump.

There are other trivial things to consider, but a mechanical system will be more friendly with the gasser you are converting. Search the forums for other instances of this conversion. There are several. Try to keep it simple. If I were doing a conversion today, what I've stated above is the direction I would go. I don't care much for the EFI system, although the one I have has been essentially trouble-free and extremely reliable.