What must be done to them to run on non pollution equipped engines- ie, the holes below the exhaust posts. Are they pretty similar to the 340 heads stock to stock? If not, what needs to be done to them so they are on par w/the orig 340 head?
only plugging the emission ports if you are not running the smog-port exhaust manifolds. As far as performing, depends on what 340 head. Generally, it's the same head with 1.88 intake valve.
