Hi Slurpee,
We have an 05' Mustang. Plenty of room in the back for child's seat, and children. I'd say at least till they hit teenage years. Our kids ride in the back of ours just fine, wife and I both have our seats adjusted back pretty far too (She's 5'9", I'm 6'1")
The GT is nice, but as of last Oct., there was a 4 month wait list on them. You're also most likely going to pay full MSRP. This might have changed by now, but do some research.
Its all about getting what you want, not what others think of your choice.
Enjoy WA state!