When you tapped for the threaded freeze plugs, what size plug did you use?
I can't remember for sure, but I'm pretty sure it was just a 1/2 inch pipe plug. We found whatever plugs we had laying around that was similar to the holes those freeze plugs were used for, then tapped them. The cast block didn't take to tapping real well using the tools we had, so we were sure to use an aggressive sealant to help hold the pipe plugs in place.
Corey