Those "power pack" heads are 70 years old, or more, and most of them have been rebuilt several times. If you can find a set of heads for a 305 4 barrel engine with the 58 CC combustion chambers, they'll outperform the power pack, and they can be ported and bigger valves installed to gain even more gas flow. They're 30+ years newer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!