They used to stick when 'new', it's got vulcanising gaskets with heat activated sealer.
It's a load of fun to remove cylinders without breaking anything.
Try a safety razor blade and 'split' the gasket as much as possible before beating it with sticks
Late 70's/early 80's Honda's were never meant to come fully apart, (clutch and generator covers come off reasonaby easy and cam cover has rubber gasket)
I'm almost surprised Honda didn't JB Weld the cases together