I think the opposite is more likely. If your building an engine and you want to get the quench and compression right you are more likely to use the aluminum heads. If you are building an engine with cast iron x heads your probably not going to the extra work and expense to
equal the compression of the aluminum heads or use a stepped piston to get the quench or have them ported for that matter. More likely if your building a motor with x heads it's because you already have them or can get them cheap. More likely a "valve job and bolt them on" scenario to save money. Of course there will always be exceptions like if they want all original or rules dictate or they are handy at doing the work themselves and like the challenge. I might find myself in that last category but usually regret it before i'm finished.