[racer]

Have you sat in one yet? One advantage of the GT3 is the center console delete.. which allows one to splay you legs a little wider. I am shorter than you but had issues in my boxster. Once I removed the center console (with the GT3 delete kit) it was mush more comfortable.

After that, yes, you would need a machine shop/race shop fabricate longer seat rails for you.

Depending on the GT3 you buy, it will either have stock sport seats or the "euro" seats. The euro seats*might", due to their (lack of) padding, offer a bit more legroom than the original seats.