A Hemi head will fit AND RUN on a lean burn block, I've done it (XR2i head)!
The pistons are different in the way that a lean burn only has 1 high spot per piston, a hemi has 2 theoretically losing compression (how much I don't know) but this can be sorted by a: replacing the pistons or b: skimming the head.
You say you have a high lift cam, when the head is fitted and the cam belt is on, turn the engine by hand to make sure you don't have any valve contact before starting.