This is my first post, I sure have found allot of great information here so far. I have what I believe is a 1971 rear end with 273 open diff, i was told it was a posi. It was added to the car purchase after the fact at no charge, however it is not posi.

I'm building a 1969 stock "C" head 455 with a stock th400 trans in a 1980 cutlass which has the stock 7.5 open rear end, I believe this 1971 rearend will be a more robust replacement. I do want to upgread to a posi with a 342 gear. This is what I have been able to find so far on the 71.

10 bolt round diff cover with slight bulge for ring gear clearance
axle tube code "GHC 202 2"
ring gear code "3910223 15:41 6 71"

Is this rear end worth keeping and upgrading or should I just start with something else?

 

The '71 rear is going to be too wide for the G-body. I believe also that, at a minimum, the upper control arm mounts are in the wrong position, close, but far enough away that they would cause a problem. Depending on your budget, you might want to hold out for a used 8.5", or a new 9". The 7.5" might surprise you too, mine has been holding up. Whatever gear is in there though, it's not going to be good for much other than high-speed cruising.

 

I think you should look for a drum-to-drum from a '78-'87 442, Monte Carlo SS, or Grand National. They are relatively easy to find, not terribly expensive, and with any luck you can find the ratio you desire. The rear from the above choices will fit perfectly requiring no modifications. With a little elbow-grease, some new bearings, gaskets, fluid, and the ever-important black paint, you can do the swap in an afternoon.

 

That's an 8.2" Chevy rear,so I wouldn't bother with it anyway.
They do,however,make control arm kits to mount the A-body rears into a G-body.The A-body rears are wider than the G-body,so you would also need wheels with deeper backspacing.The combination of the control arm kit,and different wheels,make it something to ponder,if it is worth it,or just buy a G-body 8.5" out of a 442 or Grand National.

 

If you want a near-stock differential, as noted above, try to find a bolt-in. If you are going to upgrade everything, you will have $2-3K in it and can start with anything, the 9" Ford being strongest. Quick Performance offers bolt-in 9" rears, and 507olds above has a lot of 8.5" stuff and expertise.

 

OR--- I have a custom 8.5 10 bolt custom made for the G-Body cars that botl right in...

 

or --- maybe a 8.8 ford g-body rear...

 

Yes,the 8.8 Ford rears are becoming popular with the G-body cars as well.