They're worth whatever the buyer and seller agree on. Most of those heads have been worn out, rebuilt, and worn out again, possibly several times. Other than impressing the mouth-breathers at a car show or building a period-correct high performance 327, they really don't have any advantage over any other cylinder head with a 64CC combustion chamber. I used to get 60CC chambers by angle milling the old 76CC low compression 350 heads. With a set of 1.92" intake valves, some 1.6" exhaust valves, and a little pocket porting, I was getting dyno numbers the camel hump heads couldn't begin to match.
If the seller wants more than about $200.00 for a set of 461 heads that haven't had new valves and guides installed, he's trying to rob you.
You'll need to use a pretty radical cam to hold the compression pressure down to a manageable level with flat top pistons in a 350- - - - -about a 280 degree duration with a .400" or higher lift. You'll have a premium-gas only engine that's very picky about how much timing advance you can run without detonation problems and piston damage.
Jerry
