The Intel LGA 755 cpu socket and cpu heatsink retention bracket are standardized. No problem there. The third party cpu heatsink retention brackets will fit motherboards correctly.
Are you trying to mix and match third party retention brackets and cpu heatsinks? That is going to be a problem. Even though all the the 755 brackets will fit the 775 motherboards, they probably wont fit a different cpu heatsink. The heatsinks come in a variety of sizes and the mounting method is not always the same. So yes, they are usually model specific.