Don't forget that the cams on top of the slave cylinders are notorious for corroding solid. Are the levers on the backplates moving? I agree that heating the drums may be the best approach. If you can get the drums to move at all, ease them round until you can back off the adjusters - that will tell you if the cams are jammed. Of course you need to back off the adjusters quite a lot before you can get the drums off.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978