A 4.6L will fit, although you might need a body lift to make it fit.
A stock 6-cylinder transmission will not work on the 5.0, I'd recommend a C4.
EFI is difficult, but it depends on state laws and your budget on how difficult it is. You could run a stand-alone engine computer (like what hot rods use), but that'd make your CEL useless, which is needed to pass state inspections. Standalone is much easier to do as it requires only minimal hookup to the chassis wiring.
Even a 5.0 is not a direct, weekend swap. To me, it wouldn't be difficult, but to some, it'd be near impossible.