Axle flange to axle flange should be 60 1/8 inches, Cranky is right the 8 3/4 for those years have the same dimensions. Some may have 10.5 brakes while others I have seen have 11 inch. The scat pack was 3:54 ratio and the super scat Pack meant 4:10's. The spring perches should be 44 inches center to center. I pretty sure all B bodies from 66 to 70 have the same differential dimensions no matter what engine it had.