The springs are further apart on the 2nd gen Birds. So, the spring perches must be moved closer together, for it to bolt into your 1st gen Bird.

The Lots of the X-body cars, from the early to mid '70's, such as Nova, Omega, Apollo, & Ventura 2, had 8.5 rear ends that will have the perches in the correct location. This swap will probably require a different length driveshaft. The guts of these are the same as the 8.5's that came in '71-'81 F-bodies. But, as mentioned, the spring perches are in a different location.