drilling steamholes is an absolute must if you wish to avoid major problems. about the heads ... are they as cast or are they ported ...do you know what the flow numbers are and at what lift ...also, youshould have them flowed with the manifold on to see if what you intend to use is compatible or is it resticting flow... it's of little use to have big flow numbers at say .600 or .700 lift if your cam won't see even .500 of lift.