Trailing arm rear axel perch question

[ChadW]

I have a 66 c10 that is a frame off resto currently...I am slowly working to get it together. It has a cut down 10 bolt posi rearend. My queston is when putting it back together with the trailing arms in preparation for welding the axel pads on it, is there a specific "factory" width that the pads need to be set at? or do you just get them where they seem right and make sure they are equal on both sides?

[vin63]

When I was still doing modifications to the chassis (frame and drivetrain location), I had the frame on a jig and located center marked with a plum line. I used that to locate everything, including the rearend housing.

I have done this in the past without a frame/chassis jig, by using adjustable jack stands and setting the chassis at level, dropping plum bob marks at several points to locate a centerline in providing me with the location reference.