1971 Blazer 2WD Conversion, Finding Axle Centerline?

[milehightoyz]

Converting a 71 blazer to a 2wd Have complete 4x4 71 blazer rolling chassis. and complete 87 GMC longbed 2wd chassis. Anyways, Is there any trick to finding the exact centerline of the axles before pulling them out to swap in the 2wd xmember? How critical are these measurements? I understand they should be as exact as possible, but I guess my question is what are the tolerances? how much room for error is there when getting the new 2wd crossmember centerline lined up deadnuts with where the old centerline used to be? At first was contemplating cutting entire frame rails off blazer and 87 and welding the 87 subframe in the blazer so I can get my z done at the same time, but I maybe opening up more room for error that way. Just wondering what you more experienced suspension guys can suggest. Thanks in advanced. John