440 Heads - Angled Spark Plugs?

Anyone running angled spark plugs? I know some factory heads had angled holes, but none of the aftermarket aluminum heads seem to have that option. Mine stock cast iron heads are straight plug and they work fine with the headers I've got, but just from looking at it I could get quite a bit more room with angled plugs. Wonder why none of the aftermarket heads seem to have the option.