The one problem I've found with some of Harken's stanchion mount blocks is that the screws fasten into a press-fit threaded insert, and if you're not careful, the inserts can be pulled free...making the mounting system not hold very securely.
I agree with the Dog, it happened to me. The insert partially pulled free and the plastic housing cracked.
Be very careful not to over tighten the screws... It didn't take much torque for the above to happen.