It's better if you already have to weld perches on to weld them at the correct angle, not to weld them and plan to use shims.
Assemble the axle with perches in the truck, not welded, adjust angle and everything the way you want it and just tac weld the perches in. Remove axle, check that perch angle to pinion is the same each side, cut them loose and match best average if necessary. Weld them and reinstall in the truck, should be perfect and require no shims with this method.
