You obviously don't love the car enough. Not a family car? My first car was an old Fox body, given to me by my parents- which means I was raised in that very car myself. I don't remember it ever being a problem, and I was the kid getting stuffed in the back seat. My mom raved about the hatchback all the time, as my wife does with hers. There was more than enough room for four people, especially if two of those people aren't adults yet. I fail to see the dilemma. Now, if you can't afford it anymore, that's something else entirely- but if that's the case, you won't get anything else worth buying or more affordable to maintain for the price this car will fetch. Just my $.02
More on topic- I have found that the AOD cars are actually worth more, not only because it was an expensive option but because they tend to have not been beat on as hard. The way you will get the most for it is as everyone said- pull the mods and put them on ebay, and put the stock stuff back on and clean it up really well. The exhaust is the exception to the rule, as selling a car with a new exhaust is a good feature. (notice I did not say performance exhaust- just NEW) it is also important that everything works. Unless you can't get any kind of deal having the heater core replaced, it's in your best interest to not have to tell potential buyers that it failed so you bypassed it. They will wonder what other things you mickeymoused in there because you didn't want to fix them. On the other hand, "all it really needs is a heater core" isn't bad for a stang this old, if in fact everything else works right down to the ashtray door. 142K isn't really high miles for a '90, my '91 had 155K when I bought it, and it's over 234K now. But, it will have to run well and look good. I still would be surprised if you get much more than $3000 for it, but that doesn't mean you shouldn't ask more. I'd start over $4000 and see what happens. Good luck