So I ordered a 60mm square heatsink and the mounting area to the surface is 35mm round, is that going to be enough for the heatsink? It's going to be in a vibration area.
Possibly no one said anything because they could not understand what you were trying to do, and just passed by in the hope that someone else would pick up on your problem.
As above - I didn't have a clue as to what you were trying to do. We don't have the weight of the heatsink, the temperatures involved, the vibration parameters, what you are mounting it on etc etc.
Will that allow for differential expansion of the heatsink and whatever device you're fixing to it? Do you need electrical isolation between the device and heatsink?