I'm in the process of doing this on my '72. I got third row split seats out of a 2007 Escalade. I did a lot of researching/measuring of different seats and for me I saw four choices, all of which have integrated seat belts.
1-The little passenger seats: Pretty hefty price wise. I believe the style used for 60 series fits best. The advantage is plug and play.
http://www.littlepassengerseats.com/
2-Late model 3rd row Trailblazer seats: I couldn't find any of these to get a good measurement of, the EXT model is not too common around here but based on size I think these would have been my first choice.
3-Late model 3rd row Yukon seats
4-Late model Escalade/Suburban 3rd row seats
3 and 4 are the same size but there are two styles, a split bench and single bench. Off hand I want to say width is around 45-47" which if it's not split would be a bitch to get in and out of the truck. 2007 and later models have shoulder belts for all three positions where earlier ones have a lap in the middle. The belts also don't go above the seat so it gives a cleaner look and though the same size they are more square looking which I think makes them more natural looking in the truck.
2nd major difference between Yukon/Escalade is the seat height. My reference is a 2000 Yukon XL. Where the seat hooks into the floor at the front the bracket is raised about 4". I'm just shy of 6' and when I sat in the Yukon seat in the Pig, my head wasn't quite touching the headliner but if I had more hair it would be.
So I ended up with the 2007 Escalade seat. They have no lift at the front so they are about 1" off the floor. The seats fit fine, when it's about an inch off the tailgate there's about 8-10" between the front of that seat and the back of the second row. Because the seat sits lower though, the floor is getting cut out and dropped there to accomodate feet. This means maybe moving the gas tank and the frame brace that the shocks bolt to. I plan on owning this for a LOOOONG time so I want it to fit the kids when they're not kids anymore. If you're looking to put under 5'8" back there, the Yukon style should suit you fine. The only issue I have with it right now is when they are in the upright position. They fold right up to the edge of the wheelwells before they are fully upright. I think my fix to that will be to add another row of brackets at the front right behind the lead ones which will bring the seat back far enough to stand upright. The other option is to cut into the wheelwells about 4" square to give me the needed space. Also since the sheet metal is pretty thin back there, I'm going to add plates underneath to make sure the seats are in solid. Here's one photo to show you some of the challenges. I've got a few more I can send if you want from different angles.
One other thing. I'm changing to a split bench in the second row so it's easier for them to get in and out. The seats are from a early 90's Pathfinder. Amazingly close fit but I haven't starting bolting it up yet.