Adam:
The similarity of the poses on the covers of Spellbound 3 and 22 is, I believe, an astute observation on your part, but I still think the cover of Spellbound 22 is more likely Harry Anderson's work than Heath's. Pictured below are portions of the covers of Spellbound 22 and 3, Astonishing 35 and Mystery Tales 21. All but Spellbound 3 I attribute to Anderson. The monster on Astonishing 35 is similar to that on Spellbound 22, and unlike any work of Heath that I have seen. The pose of the startled man on Mystery Tales 21 is similar to the man on Spellbound 22; also, and perhaps most persuasive, in my estimation, is the lack of the dramatic shadowing that is one of Heath's hallmarks.
What say you?
As an afterthought, I believe that argyle socks are typical of Anderson's work.