What modifications need to be done to the rear diff. from a 77 Firebird to fit under a 69 Firebird?

Performance Years sells the spring perches you will need, remove old ones, do your measurements & weld into place. Make sure the perches are in proper position when installing, look at your old rear end for reference. There is a slight angle in the perches thar relates to the spring angle in car. Keep in mind that the '77 rear end is wider than the original. Driveshaft will need to be shortened approx. 1" & balanced to work. The 8.5 will be much stronger than the original 8.2.

Steve

I knew that the `77 diff.was wider,I justwanted to make sure I was was totally right before I started to chop up the housing,it`s in too good of cond to be experimenting on.The 8.5 is not only stronger,but much easier and cheaper to buy parts for.