if it's big enough to cover the GPU, then just arctic silver epoxy it on. is it is a big heatsink, you can drill mounting holes and use 5mm screws to out it on (just make sure that you have plastic washers between the PCB and the board on the back side to keep the head of the screw from scratching the PCB). put some asII on the GPU, and voila. since orbs are mostly just for looks, i think that the socket7 heatsink would work really well.
--jacob