I would agree with the advise that 6 cylinderovertexas is giving you.
http://www.dennysdriveshaft.com/app/webroot/img/File/how_to_measure_diagrams/b.html

This is exactly what I did in my 51 Chevy shortbox build. In my case once the rear differential and transmission were in place I measured the exact distance using the above link as a guide. After very careful measurements, the 93 Astro shaft was taken to the machine shop cut,balanced and welded back together. Once I got it back everything fit very well.I've been running it for about 2 years now with 0 problems works very well. Hope this will help you out.