Measure all sides and every part of the heat sink to get the full surface area then check out growmau5s heat sink vid on youtube to get the formula to give you a better idea of what it can handle, its only a rough guide. I'm too lazy this morning to look it up.
It'd take a little bit of digging, but about two or three years ago Supra was initiating a lot of discussions about heat sinks and what they could handle. Might be worth checking out.