A problem with applying heat is that aluminum is a great heat conductor and heat sink. That being the case the heat you so carefully apply to the screw is rapidly going to transfer to the aluminum mount. I think Mike Bonner has the best idea, carefully drill out the old screws and retap with larger screws. Or you could carefully drill the heads off the screws, and then with the scope safely removed from the mounts you could use a butane torch to apply heat directly to the screws and grip the remaining screw stubs with vice-grips to remove.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)