sorry to speak in both english and SI units, but I'm bilingual. I've measured quite a few boards and found mounting holes anywhere from 64-67 mm apart. almost all were exactly 36mm in the other direction.
I drill 3/16" holes (4.76mm) in my WBs for mounting, which has (thus far) fit all motherboards when used in conjunction with #6 (0.138" or 3.5mm) mounting hardware. 6mm holes would give you more room for error, especially if you cannot locate the holes accurately. Hope that wasn't too confusing.
How thick is the copper base on that block? I'd say 1/2" should be plenty thick for that design.
I drill 3/16" holes (4.76mm) in my WBs for mounting, which has (thus far) fit all motherboards when used in conjunction with #6 (0.138" or 3.5mm) mounting hardware. 6mm holes would give you more room for error, especially if you cannot locate the holes accurately. Hope that wasn't too confusing.
How thick is the copper base on that block? I'd say 1/2" should be plenty thick for that design.