340 issue after camshaft change

Don't be so quick to condemn this. How have you "made certain" the valves are closing? One way is a leak down test, and along with that is visually cycling the valves. Crank til one valve FULLY opens, then visually turn the crank ONE TURN. This will put the lifter "for damn certain" on the heel of the cam. See that the valve lash is "loose" and that the valve IS CLOSED If you don't have a leakdown tester, at least visually cycle the valves both closed and pressurize the cylinder at that TDC and listen for air out the intake, exhaust, or cooling system.

Having said that we DID have one member on here (273 I think) got ahold of a marine counter rotation cam