FWIW, just one (1) vintage way:
Have successfully removed quite a few heads which sat out of doors for over 10 years as follows:
1. Get an ice pick or similar tool to dig out debris/rust around head bolts after removing head bolt nuts.
2. Get Play-dough & build 3/4" high circular dams around all head bolts to "contain" fluid penetrating oil -- i.e., without a dam, soaking overnight translates into 3 minutes after the penetrating oil evaporated on the head's surface; however, the dam can hold liquid while you sleep overnight -- this is true soaking over night.
3. On all sides, with a screw driver, carefully & lightly tap on head gasket to mash it inwards about 1/16".
4. Cut about 25 small hardwood wedges, (hard maple, oak, ash, etc.,) with very low slope angles, & drive same between block & head with a small hammer.
5. If wood wedges get deformed, cut more wedges -- they are inexpensive.
6. Remember not to go too deep where valves occur.
7. Something will sooner or later give.