I sell a lot of Mustang parts on Craigslist,most people are nice and a few that are out of their minds! I never respond to emails and tell the person I'm firm on the price before they arrive
You will need the wiring from the 91 including the use of the complete dash and the steering wheel (the whole shaft also) or find a 1988 cali mass air computer harness or a 1989 mass air harness
stock bolts for the upper seat have a 7/16 head with washer and are 1" long with fine threads, the carpet is held on with the plasic clips and philps head screws
IMO I think your computer is starting to go, I had the same problem with my 1989.I changed the ign switch, fuel pump relay,and checked everything.I put a spare computer in the Mustang and it started right up and is still running
If all else fails, and you trust a repair shop they can put a judgement against the car and get a title.If not you could buy the Mustang and use all the good parts on a 4 banger body, I would get the vin # first and run it