Some good info here.
http://www.hamtramck-historical.com/dealerships/1968DealershipDataBook-06.shtml

The 318 was the largest engine you could get in the GT. I think the 8 3/4 could have come in just about anything. It seems like there is no rhyme or reason to what A-bodies got the 8 3/4. My 318 vert has a 7 1/4 but I've seen /6's with 8 3/4's.