I need to know what is the largest valve size that will fit in a 440 head, casting number 2780915. These are the closed chamber heads from 1967. I want to port these heads with much larger than stock 2.08/1.74 valves, in an effort to make well over 300cfm. This will be going on a 500 cu. in 440 stroker engine with all radical race type components, and is currently in the works.