go to www.6mmbr.com
and look at recommended smiths. Any of the 1000 yard smiths will have them. I would go to Bruce Baer at www.baercustomrifles.com
as my first choice though, he has the most experience with blocks and testing for them.
couple points. You definetly want a split block instead of a glue in. Barrel changing is a whole lot easier and cheaper. do not use a block over 9" in lenght. that has been found to be the optimum length.