Maybe you meant 69, not 79? There was no GS455 in 69, just a GS400. 64-72 A body rears will bolt in. The 64-67 and the 68-70 Buick rears were 8.2, but the 68-70 is different than the 64-67 8.2. Some of the rears were a bit wider than others.
Yes it will fit. Just need to mod the spring perch to a raised one with 2.25" tubing we extend alot of the G-body perches as they are too short for circle track etc. But yes they will interchange. Jim