Some ifs here to consider. "What ifs" actually. What if the guides in the iron 915 heads are still good? Possible but unlikely But if they are still good that saves about what $300 in machine shop costs? Its possible a good set of street heads can be made from a vintage set. I mean when were talking an honest 350-400 hp home build these heads are more than capable. Factor in a DIYer sensible porting job and you have some really great heads. Just things to at least point out.