Robert,
I made my own engine stand for my IO-320. The hole for the main crank is 2-5/8". There are then six 5/8" holes at 2-3/8" OC from the center of the crank. They are evenly spaced around the flange so that's 60 degrees for each bolt hole.
Hope that helps. If you're doing it out of metal you might want to add a 1/16" for a bit more room. I remember mine was pretty tight, but it was wood and pliable.