I also used the Pacific Fab/Tilden crossmember. Maybe a different company now?
http://www.pacificfabrication.com/
Have an LSA and adapter from Advance Adapters for the 6L90 to 205. The 6l90 is a bit longer than most trans but new adapter is shorter than stock one. Wanted to keep motor farther back and needed new drivelines anyways. So I needed to move the stock transmission crossmember back almost 2".
AFAIK there is no difference in the 4x4 firewalls. Maybe the 67 trucks are different? Would like to know what the difference is by suburban99.