It depends... is your mustang a manual or auto? If it's an auto V6 you can toss on some nice dampers and camber/caster plates and I doubt you'll ever need to show them receipt/books. If it's a manual... hard to say how far you can push your luck. They won't look under your car unless your blatantly breaking the spirit of Lemons. At most, if it's lowered, they'll sit on your car and bounce it and immediately get an idea if you have cut springs or "real" suspension. Real suspension can be anything from sleeved bilsteins to Ohlins... but they likely aren't going to bend down to look. With that said... if you go OEM dampers, they likely won't even pick up on them being replaced... and if they do you have a good argument for it being a safety repair if they're rusty. It's not technically a safety repair, but I think it's a fair argument, especially since you're not running a BMW or a miata. We run sleeved bilsteins in our Miata, but it's because we had enough spare parts to piece that together for free.99 and we didn't have any stock suspension (car came without suspension). That was the cheapest way to get our miata rolling, and I make that argument every time during BS and they let it slide. Look around craigslist/marketplace for some used suspension and that may make your decision.
Idk mustang suspension very well... but there's usually ways to adjust camber and caster with some home depot hardware and fabrication.
Full Ass Racing
#455 Piñata Miata - 1990 Miata
#735 BMDollhÜr 7Turdy5i - 1990 735i