After a lot of surfing on the boards and elsewhere I have learned much about the types of rears that came in our Montes....My 70 came with a Pontiac 8.2....My build sheet reads....rpo code GU7 2.78's and I checked the #'s and codes on the rear which definitly are Pontiac...WC = Tempest/GTO/f-85 w/2.78..What everyone should know in order to upgrade this type of rear is this.....The BOP 8.2 Is definately a bolt in axle rear That has the 2 scallopes on the cover...There are "27" splines on the pinion and "28" splines on the axle....Auburn makes a posi unit with cone type mechanisms...this unit is the least expensive and cannot be rebuilt...Eaton now has a posi that uses friction plates instead of cones which is rebuildable but more $$...The ring and pinion for these rears are also more $$...Installation kits run the same for all types of rears....I found a package deal here in NY, Bohemia L.I... Eaton posi, ring and pinion(3.55,3.90,4.11 or 4.56)... no 3.73s and the install kit for $750.00....This isnt cheap but if you whant to keep your car #'s matching its the way to go.....One more thing if you are looking to go with 3.55's or bigger the carrier must be for 3.36's and >.....I hope this helps..I know I repeated some info here...I'm just looking out for all with this type of deal....Al