IRS Differential Output Flange Height

[snapper]

For your application it would be easier to mock up the rear by attaching the rear wishbones and uprights then using a piece of steel tube in the place of the shocks set the bottom wishbone level, then run the drive shafts level to the diff, this sets diff output centre line, you should run the diff input flange level but it can be off centre left to right, Sierra diff is off centre left, right by quite a bit, the diff may even be ok nose slightly up.
Prop shafts don't run straight but usually slightly off line apparently this is easier on the uj's