From my quick searchs, GM has used three different shaft diameters...
Small HEI is .428
Full size HEI is .490
BBC HEI is .500
I think?
Edit: Goal is to stick something like this
http://www.summitracing.com/parts/ms...4550/overview/ in a vortec housing.
Edit #2: Measure vortec shaft, .428 it is.