All the seatbelt bolts are T50. They go straight through the body so make sure to spray some PB Blaster, Liquid Wrench or the like on them before you take them out. They're hardened and are tougher than the T50 socket oftentimes.
When i took the seats and belts out of my truck I twisted off a Lesile T50 (returned to sears and got money back) and also partially twisted off a T50 from a Husky (Home Depot) set (got my money back). I ended up with a craftsman one which I finished the job with. Craftsman also has the great free replacement warranty which you may need here. I'd recommend snap-on if you can get it. I had to use a 1/2 in breaker bar and a lot of my weight to get them free. When you're doing it it feels like you're reaching that scary point where the bolt is gonna snap off, but the socket is much more likely to go. An impact rated socket and impact gun would prolly be easier. Put some anti-seize on them when they go back in to make it easier in the future.
If you're taking out the seats too, the regular seat bolts arent as tough as the seatbelt bolts and they will snap easily. Use a lot a penetrating oil. To get to the back bolts, put the seat in the normal position (not folded), move it up all the way (with the lever under the front) and then fold it. You will get plenty of access to the rear bolts.
-Ted
