built up 318 oddly low MPG's? hp rating?

I agree, swap the heads and be done. 66-67 273 heads should be easy to find, nobody wants them and will give you better compression than anything else. If you do have a vac sec I'm sure 650 can be tuned to work fine. I'm sure that intake manifold is fine, def don't want any type of single plane on it.

I just had my car's original 273 heads re-done and swapped them for open chamber 318 heads that were on the newer engine. That, plus porting, and the xe268 cam really woke it up but I have not driven enough to check MPG.

According to my measurements, I gained almost a point of compression, I'm at about 8.56:1 now by going to the 273 heads, milled .015, and Mr Gasket 1121G head gaskets (and that makes me think you're probably 7.5:1 or lower, depending on the actual chamber CC and what head gaskets are on there).