Ah, makes sense. I have put a Blazer seat in a Burb (not for the tumble, it's just what I already had) and it fits fine, but the Burb did not have 3rd seat to begin with. The front bolts down to the floor, so holes are needed for that. I'm not sure if the rear latch has the same spacing or not, if it quits pouring down rain, I can go measure.
There is a big drawback to having a Blazer seat in a Burb. In a Burb you can fold the middle row flat and remove the 3rd row seat, leaving a very large and flat floor for hauling a lot of large stuff. When the blazer seat is in there, it's in the way of large items and you can't just quickly take it out of the truck.
Have you thought about maybe creating a way to fasten down the seat with it laying in the "tumble" position? Like a matching color GM seatbelt bolted to the floor. Just loop right over the top and buckle it.