I'm not all that familiar with the fine points of the 924/944. However, you might want to be 100% sure that your car would actually be legal in GT. I was researched running a car of similar layout in GT a couple of years ago, mainly based on the loophole in the rules for engine swaps in the GT classes. However, the rules guys informed me that my car could not run in GT, because of the rule that reads: "Body styles produced with jump seating for more than two people like a Datsun 280ZX 2+2, Porsche 911, or Honda CRX, even though they may have only two seats, will be considered a Coupe and Sedan and must run in that class. I did a quick search and found pictures of 924/944 upholstery for what certainly appears to be a rear seating area. I doubt if the fact that it may only be comfortable for double-amputees would count. At any rate, I'd suggest you get something in writing from the rules committee before you spend any money and/or get protested.
Jeff in Boise