Hi, I am new here at the forum (located in Sweden) and I have a deville coupe 1970, I want to put in a diff locker or any kind of posi, spool, non-slip, lock-right locker....but it seems to be difficult to find.
Will for an example Powertrax no-slip fit:

jegs.com/i/Powertrax/754/92-0788-3005/10002/-1

Is it 8.875, 30 spline inside my rear axle?
It is 12bolt on the cover plate.

 

Welcome to the Cad Forums!
Wow...that's a good question....don't have an answer, tho.
Perhaps a differential out of a full-sized Chevrolet(Estate Wagon or similar) will work, as I know some of those did have Posi-Trac diffs.
We may need to do some research on this!

 

Im sure judging by the date that you have already found the answer to your problem, but for everyone else, according to the GM manual, the 1970 caddi's had to rear end options. one was a normal rear, the other was a system that used clutch cones to direct the traction to the wheel with the GREATEST traction. I cant remember what the system is called to save my life at the moment, but it is not a limited slip dif, but is very similar, except it doesn't lock the rear together. These are only identified (besides crack'n em open of course, by a metal tag attached to a bolt holding the case on. Standard ones are all 2 or 3, while the ones w/ this traction control have a "G" preceding them. ie, G2 or G3.

 

hey i just put a 1995 impala ss axle in my 1970 cadillac. the axles are the same in the 94 to 96 impala ss's. its a little less than an inch narrower than the caddy rear end. you can weld to the case which is a total plus. it comes with matching bolt pattern (though different thread count) disc brakes and a posi to boot. its a gm 8.5 inch rear so i now have access to a universe of aftermarket products. as far as sitting lower ive had no problems with the rake or anything of the like, though i did have to fork over a extra 400 for a custom drive shaft. im now running 4.11 gears and a tru trak limited slip and couldnt be happier. except n long drives which this car isnt really great for these days (cost of fuel) anyway. everything said and done zero to rebuilt impala ss gm 8.5 rear installed and running im under 2000. including all new bearings gaskets gear set rotors pads calipers lines parking brake cables it came with the true track limited slip which was like finding gold but we all get lucky sometimes