What you describe seems like an exhaust leak somewhere. The stock webbed headers regularly crack, get welded and crack in a different spot requiring another weld, until most owners get tired of chasing the problem and replace them with a better built set. This can also gain horsepower at the expense of a bit more external noise, not in the cabin. So the car needs to have a really thorough exhaust leak-check done. I agree, an exhaust whistle can be annoying, and if thats what it is, it will only get worse and maybe change tone.
Normally I would suggest you simply borrow a stock muffler from a Pantera owner that has upgraded his car and has stockers laying behind the garage. A minute or two of running would tell you if its something in the completely welded muffler that's caught or broken free. To fix, the muffler is cut open and the whistle-producer is eliminated. Or its something to do with the header or connecting pipe. You might get lucky and it's only a loose exhaust clamp. Of course, all this is based on whether you actually complete your purchase.
POCA has around 900 members and some have more than one car. Members regularly swap parts around for trouble-shooting. But you're in the N. Detroit area, well outside recognized hotbeds of Pantera activities (FL and So-Cal), and shipping bulky parts around the country on a hunch is not cheap. Shops that specialize are scarce in your area. There's one in NW Ohio (Amerisport) not terribly far away that could diagnose and probably fix it in a few minutes, run by Kirk Evans. He's no longer a POCA member but he does post to this forum occasionally.