Body Alignment?

Hey all,
I am beginning to think about how to put my 5 door 109 Series 3 back together. Unfortunately, the pillars in the bulkhead are completely rusted through so we are going to have to replace them. We have new b pillars to put in too.
Any tips on ensuring that everything will look straight and lined up once finished? Has anyone had this problem before?

You don't say whether you've removed the bulkhead? Because its best to leave it in situ and align the new Lower A post fittings with the outriggers either side! Temporarily fit them with tack welds and ensure that BOTH doors line up and open and close. Remove doors and weld properly

Another thing to do is measure from bulkhead to front of back box at the door openings. if the posts are rotten there's no guarantee it hasn;t shifted already, so part of replacing them would defintely be checking that alignment - also checking for vertical shift/sag.