I guess if you have a CNC machine to threadmill the threads, then you can make whatever threads you want. That is a cool way to do it if your machine has a lot of Z travel. My HAAS only has 16 inches of Z, so I don't think I could get the block on the table and have any room for a tool.

I might suggest a 1-11/16-16 UN threads. The 1-5/8" through hole would work great with that thread and leave 75% threads.

http://www.efunda.com/designstandards/screws/unified.cfm?start=397&finish=472