[blucar]

I assume that the question about aluminum heads for a 21 stud engine would be 'the finned type' not something similar to the stock '35-36 Ford aluminum heads.
I would surmise that '37-38 21 stud high compression heads would be in the unobtainium category. Due to the tough times of the late '30's very few company's would be making speed equipment for such a small select market. After WWII ended most performance minded people would be re-powering their cars with the much better 24 stud engines.