I just wanted to chime in, since I've experienced both motors. I had mustangs for the longest time, and while parts are cheap, and there are a lot of things you can do with them, you get a lot more car with the f-bodies. My roadmaster has the same LT1 as the Impala SS's(iron head, 260hp), and I have been shocked at how well it has responded to minor mods. I was happy right up until the optispark went. That was at 116k miles, though, and yes when you look at it you will feel that the GM engineers must have been racing to meet a deadline or go to the bar! Unless you get a newer mustang, the brakes are shit, the F bodies come with 4 wheel disc, the mustang trannies are shit, the gm's are better stick for stick or auto for auto. The interior issues, well that's a different story. If you keep it in the garage and it doesn't see drastic temp changes, your dash should last. I have seen them split in one day going from frost to high 70's as we do here in Fl.
I kinda feel wierd saying it because of all the time I spent with mustangs, but get a formula or TA(Z-28's look like Geo's on Steroids ) You'll probably like it better, plus a 150 shot and some tires you're going low 12's, high 11's!