Not all GT's have the mounting holes in the same area. I think the Ebay belt is a good price, but you might need to weld in a mounting plate behind the rear side panels for the retracting part of the seat belt system. I guess an alternate method would be to cut out the shoulder mount and hang the retracting part were the shoulder mount usually goes. Here is one that I almost bought this week except the price is per seat and not for both seats.
AMSAFE Retractable Lap & Shoulder 105" No Shoulder Mount