FYI, just in case you aren't aware - these blocks are generally meant for holding a frame flat on a table. While they're popular for rolling dents, this isn't their original function.

As such, never rely on the built-in screws to provide any clamping force; only use them to align the blocks. They will strip the threads out of the other half of the block if you apply any torque past "comfortably tight."

-Kurt