Listen, I agree totally with you that pentroof valve covers are more appealing than the standard SBF valve covers. I use them quite a bit myself as they give more room for "race stuff" underneath. They even make them for Cleveland heads:
However, the adapters are made for the sole purpose of trying to make an engine look like something it's not....to fool people who look at the engine into thinking there's an FE under the hood. That's what I have a problem with. If you want the width, height, and appearance of an FE to make your replica look more correct, then my point was to just put a stinkin FE under the hood to begin with and you won't have to go through all the cloak and dagger junk.