Open chamber heads are that way so ChryslerCorp engineers could lower the compression ratio and by doing that limit NOx formation. I don't think there's an iota of flow difference between a 915 and 906.

I'd say as long as you're not planning any high performance work with the engine, go ahead and use the heads. They won't flow like ported 915s but they probably are at least as good as a newer set of unported heads. And, as you don't intend to use this engine a lot, I'd say disregard any advice you may have gotten about installing hardened seats. I've put 70K miles on my '64Dog with solely unleaded fuel and the valve clearance hasn't closed up a bit. That would have happened if there was valve recession with my solid lifter cam.

I recently saw some flow bench numbers on this site which included flow for 516s and they weren't that bad, probably around 20 cfm less than a later head. While that is 45 hp or so it isn't a deal-breaker as far as I'm concerned. The heads you have should be good for 400+ hp.

Using a Performer intake will get the torque in low, although I'm surprised someone hasn't told you how it's not a real high performance intake.

Your other heads are 346s, which came on a 1970 engine. Nothing special about them.

You will have really low compression until you get different pistons. Use the steel shim head gaskets to not give more away.

As far as cam goes I'd suggest a tight LSA with moderate intake lobe.

R.