The stock exhaust is going to need to be replaced, regardless, period. At that point, the "survivor" status goes out the window anyways, as I understand, so it doesn't really matter. I'd run dual, myself, but I had a customer a few years ago when I was doing exhaust who had a 73 440 New Yorker and to keep his collector plate status could NOT put dual exhaust on it, as it didn't come with it (I believe in in 72 they still had it, but not in 73). He wanted a little moreOOMPH. So, we ran 2.25" to the "Y" and then ran a 3" single the rest of the way. Kept his insurance company happy, and satisfied his performance but enough.

Oh yeah, I know it's pointless, as it seems you have made your decision, but Sheldon's got it on the $. The Chally is a new car that looks like it'll be around for a few, AND will start to depreciate, whereas the Sat is gonna hold steady at worst, and possibly increase. You can always buy a DD Challenger later!