If it's running with the truck's EFI components on it, and it's from a Mercury, the odds are it's another 5.0L V8. They were used in the Grand Marquis though 1991, and the Cougar through 1995.
@Noobz347 is right though, changing it over to a carburetor might not help anything. If the problem is somewhere other than the fuel system or electronics, it will still be there when you start it up with the carburetor, and possibly intensified since there won't be a computer reading conditions with sensors and compensating with changes in timing, air, and fuel.
If you REALLY want to do it though, there's a pretty long list of things you'll need/want to do.
First, if your truck is running the 2 or 3 fuel pump setup (most 80s through 90s F150s used a low pressure pump in the fuel tank or tanks, and then a high pressure pump on the fuel rail), you'll need to eliminate the high pressure pump, and install a return-style fuel pressure regulator before the carburetor and hook it up to the existing return line from the EFI system. The "low pressure" pumps in the tank(s) run at around 10-15psi if I recall correctly, and you'll need to step it down to 5-7psi for most carburetors. This is the easiest way to address the fuel supply, but there are two alternatives:
1. You can remove the tanks, take out the pumps, rig a different sump, change the timing cover on the engine, install a fuel pump eccentric on the cam, and install a mechanical pump and run lines to it and from it to the carburetor. This will involve a LOT of work, including removing all of the accessories on the front of the engine, draining the coolant and oil, and running new lines/hoses. The only real advantage here besides no wiring, is no need for a return line, you could just cap it off.
2. You can remove the tanks, take out the pumps, rig a different sump, and mount a low pressure electric pump like the Mr. Gasket pumps most parts stores sell where the high pressure pump used to be. The downside to this is that the pump will have to work fairly hard to get the fuel out of the tank(s), will be fairly noisy, and will be relatively short lived, but extensive plumbing won't be required and you won't need a return line (could just cap it off).
Next, you'll need to establish whether or not you've got a roller cam (It will be easy to tell when you take the intake off. If there's a metal piece with 8 "arms" holding down 8 wishbones on the lifters, it has a roller cam. If it doesn't, but has two unused threaded bolt holes in the valley, it's a roller block running a flat-tappet cam. If it has none of the above, it's a flat-tappet block.) If you do, you'll need a non-computer-controlled distributor with a STEEL distributor gear (Or phenolic or bronze, but both of those materials make it a maintenance item). If it doesn't have a roller cam, you can run any material distributor gear you want, but cast iron or steel would be recommended.
After that, if you've got an automatic transmission with overdrive (most mid 80s and newer F150s did), you'll need a TV (throttle valve) cable. Lokar makes a pretty decent, but pretty pricey kit for this. You'll need to follow the instructions TO THE LETTER, or you'll be adding a transmission rebuild to your "to do" list.
After that, you'll need to decide how street legal you want to keep it. If you're in an area that does any kind of emissions testing, you're screwed already, there isn't any way you're going to pass so much as a visual check with a carburetor on it in an emissions-testing state/county. If you're not in such an area, you've got to decide just how much you want to hurt the air that you, I, and everyone else breathes. Are you going to keep your catalytic converters? Your smog pump? EGR? PCV? Making all of these systems work with a carburetor will require careful parts selection. You'll need an intake with EGR provision (Ford actually had a nice aluminum one on early 80s Mustangs with carburetors, or Edelbrock makes one) if you want to keep that, as well as a 70s/80s Ford EGR valve, ported vacuum switch to control it, etc. Switching to a carburetor is already illegal at the federal level, but it's not really heavily enforced outside of emission-controlled counties and states, and neither is all of the rest of it (which is likewise illegal), but you just need to be aware of it.