axel housing perch location

building up an 8.75,, right now the axel housing is empty,, no axels or center section in it,
while it is that way I want to have the spring perches relocated to match up with off set spring hangers.

I have read a couple of postes that the best way to relocate the perches to get the best pionion angle is to have them not welded, assemble up the leaf springs and axel, get the angle set, tack the perches, then remove the axel and finish the weld.

I don't have welding capability so I will need to take the axel housing to a shop to get the existing perches removed and new ones moved in and re welded.

question,, is there anyway to know that the existing perch location, for angle is correct?
or do you just have to believe that it is and duplicate the location, then use shims as necessary..

anything else that I need to be asking about when looking for a shop to re locate the perches??