How to adjust or center the rear end on 1968 Chevy C10

[HeavyHauler]

I recently lowered my 1968 Chevy C10 Shortbed 6 inches in the rear. It is a coil spring rear and I installed a new double adjustable track bar. I installed 5 lug cconversion axles along with rear disc brakes. I dont have wheels for it yet so it is sitting on jack stands.

From what points can I measure to get a accurate number as to how much the rearend needs to be adjusted left or right?

Also do I have to remove the adjustable track bar then center the rear end and reinstall the track bar OR can I just leave the track bar in place and adjust the rear by letting the rearend hang and adjusting the nuts on the track bar?