Putting GT rims on a V6 is no problem at all...at least for the '99-'01s (earlier years too, probably). I've got the Premium GT wheels (AKA Bullitt rims) on my V6 coupe...they bolted right up.
Your speedometer will just be a *tad* off. It's so minor, it's not even worth worrying about. Something like 1/2 mph. But one thing I have noticed since I changed wheels...my gas mileage has decreased slightly. With the 15s, I would get about 290 miles out of 13.x gallons. Now, out of the same amount of gas, I'm lucky to get 275 miles. (I could probably also attribute the mileage decrease to running the A/C all summer...so don't lose hope)
BTW, where in Maryland are you? I live in the Annapolis area...have a Rio Red '99 coupe.