Is it an oroginal 5.0 efi car?
Most 87-93 carb swaps start life as a 2.3L mustang. Most just figure a carb is easier than changing out wiring and such.
First step is to identify what the po removed. The obvious things are the upper intake, injectors, 87-89 engine harness, and deciding on mass air or speed density. But its hard to lost other items without knowing what fuel pump is in the tank, or what fuel lines were used.
Some guys do hack conversions to carb, and some do excellent jobs. Until will know what your car is, hard to list everything really.
Gas tank with in tank pump, hard fuel lines going from tank to engine bay. Upper and lower intake, fuel rails, fuel injectors, various sensors used for efi, computer harness, computer, coil and efi distributor... I'm sure I'm missing some stuff but that's what I can recall off the top of my head
Can you find a rusted POS for $500 and use that as a parts car... once done sell extra parts and get some money back...
An entire new gas tank? Wow
FML this blows
Just out of curiosity, what's your issue with the carb?
Keep in mine, the only engine harness that will made up to an 87-89 body harness is the 87-89 harnesses. SO if you find a MAF harness, it NEEDS to be a 1989 MAF harness (or 88 cali) otherwise it won't mate up to the internal body harness and you'll need to swap that.
2.3L and 5.0 body harnesses are the same.