A little update for everyone, I stated If you use a later second gen driveshaft like I did, you will need a conversion u joint at the rear. the Precision U joint p/n is 372.
Wrong! The drive shaft I had was actually from a 73 Firebird. I turns out that the rear has a 1310 u joint and the front has the 3RL/S44. Weird, but that is what is in there now. Sorry for the confusion.

So the 1310 is also the stock u joint for the early 8.5 inch rear end, very easy to find!

Now, my pinion yoke is worn, which is someting to look out for on 40 year old 10 bolts! It is out by .030!!! So the drive shaft was wobbling around in there. I found a temporary fix by using some shim material and checking the run out. I have 30 though of shims on one side of the u joint cap and it is going quite well. Summit will be delivering a new pinion yoke to me tomorrow.