How can I attach a heat-sink to legacy ICs?
From what you describe, you don't want the heatsink to be removable. For that reason I would suggest using Arctic Silver binary thermal adhesive:

http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm

Note this is the Thermal Adhesive, not the Thermal Compound.

It's good for more than 150*C, and is one of the most popular thermal adhesives I've come across. You can buy it on Amazon for a relatively low cost:

Arctic Silver Thermal Adhesive

Good luck!