My memory is fading, but I'm pretty certain I drilled and tapped mine for 7/16-20, same as SBC. I drilled "deep enuf!" and used a Grade 8 bolt and USS washer to retain the balancer.
It works great for installing the damper too, making it possible to R&R the balancer without removing the rad.
http://www.homestead.com/chuckthetruck/Page21.html
