[PONTIAC LARRY]

Its really a wildcard on cost. If you do all work yourself and a bit of luck finding say a 4 speed 3.73 Posi 2nd gen Z28 rearend for a $100 you could use the gears and posi unit with your housing and axles. Cost will depend if the existing used pinion and carrier bearings on the donor parts are in good shape and often they are. The condition of the race will tell that tale plus the rollers on the bearing (pitting) . Could be $200 easy if yanking gears in one place and posi out another with ones ready to reinstall.. add supplies and it could run $100-$400 on a used gears, used 28 spline posi to get it together. New bolt in axles 30 spline (if upgrading posi units to one from a 90's Impala / Roadmaster) will run 400-500 bucks but if your 75 GP is C Clip you can get them for like $250 and by going to 30 spline axles you are heading the right direction to handle big Pontiac torque. Now on the 96 Impala rear swap if its really a bolt in (I would have to check that) it may be the best deal if the control arms fit properly but 2.723or 3.08 is likely all you will find. I will say my Dad has a 92-94 ish Buick Roadmaster Wagon and it has a towing package with 3.23 Posi ! 8.5 rear... I think the wag is wider though.. All new your looking at over a $1000 in parts including new axles . So used gears used posi and new 30 spline axles COULD be done under a grand if you make decent deals all new and your reaching towards 1500. Odds of finding a 73-77 3.42 posi for cheap is rare but I did it a while back ..got a bolt in axle one out of a 73 GMC Sprint