I know this was originally a post for a 235, but, I have an idea for a 250-292.

Why not take two 181 Mercmarine 4 cylinder 8-port heads and cut one cylinder off each end. Then weld back together.

Bigger valves, individual intake ports, all bolts line up and maybe even working water passages for the street. Not cheap, but probably cheaper than an aluminum head and looks stock on the street.