It is not the 318 part that is wrong. I will take a 318 over a 360 any day. Sounds like you have been chasing this down for a while. I was working for a small Chrysler Plymouth dealership till about 75-76. I remember when the Chrysler Cordobas came out. What a sweet cruising car that was. A 318 would run down the road, no sweat. Your problem is probably the Lean Burn system. My only suggestion would be to richen the jets in the carb a couple thousandths. I much prefer the carter 2 barrel carb from about 1973,
https://www.allpar.com/attachments/bbd-six-carburetors-jpg.74706/. You could adjust the position of the metering rods with a screw under the top plate. I would figure out what is wrong with the 318 before swapping heads. Your 302 heads sound good, not sure you will gain much without relieving the chamber. Sounds like you have a handle on the rest.
