What 8 3/4 rearend do I have?

Well, the first thing you can do is check to see if the bolt pattern on the axles is 4 inch or 4.5". A body 8 3/4 rear ends only came with the 4 inch bolt pattern. I just measured the inside to inside of the spring perches of the A body housing on my bench and it measures 40 1/2", leading me to believe that you have yourself a B-body unit from a 69 or 70 car. B body rears from pre-71 cars measure 44 inches from the center to center of the spring perches, where they are typically measured. The 489 case replaced the 742 case during the 1969 model year, so if your rearend has not been altered and is in as removed condition, I would say that it is a late 69 or 70 B body piece. It could still be used in your demon, but you will end up with LBP axles and offset rear rims, like a lot of guys are doing here due to the elusive A body units. Trouble comes in when your fronts are still small bolt!!! Unless of course that has been changed out already. Hope all of this helps!!! By the way, is it a sure grip??? Good luck with your project, Geof