The 1967-1969 10 bolt 8.2 firebirds are the same width as the 10 bolt 8.5 1970 -1981 firebirds the only difference are the perch center line.

The Nova 1972-1976 8.5 10 bolt rears are 1" NARROWER than the fire bird 10 bolt 8.2 and also have a smaller 1310 yoke for the u-joint. they do have the same spring perch center line and will bolt into the firebirds 67-69.

BOTH camaro 1970-1981 and the nova 1972-1976 8.5 10 bolt rears will require the drive shaft shortening to fit your firebirds from 1967-1969.

We do this spring perch relocation all the time and have used chevelle 68-72 12 bolts with leaf spring perch as a firebird rear also.

Hope some of the info clears the air on the diff swaps...if you need more help or info just ask. Jim