[chevy_mike]

I did the same swap using a '73 rear end. One thing you need to do (in my opinion) is to remove the springs. You don't want any affect they could have influancing things. Also, you never said what panhard bar you are using. If stock, you'll have to fab a mount but do that after the rear end is centered. I used an ECE adjustable bar, which connects the axle side to the trailing arm/axle mounting point.

With the frame up on jackstands, put the axle approx where it will be. Rotate the arms up and loosely put the replacement saddles (I am assuming you are using replacement ones and not cut off your old ones, that will make it difficult to get correct measurements) and u-bolts in place. With the arms in a natural "at rest" position, measure from the backing place to the u-bolt. Check both sides. You want this measurement to be the same WITHOUT trying to pull the arms over. You want them to stay in a natural at rest location and get the axle centered. Once centered, set your pinion angle (I used the same angle as the stock rear end). Now you can double check all your measurements/angles and once correct, tack your saddles.

Pull your rear end out and burn those saddles in. As a side note, I had also replaced the front bushings in the arms so they would not have 40 years of slop in them. This helped to keep the arms in the correct plane. Setup your panhard bar AT ride height and you should be good. Springs are the last thing to go back on.

You can check my '68 build for pictures showing what I did. Good luck.