Depending upon the purpose of your engine, if you really do have a good set of J heads, you don't need a replacement set of heads. I'm shipping out a set of J heads next week, for a street 440 ci small block, that are flowing as much @ .550" lift as a set of Big Block OOTB Edelbrock RPM or Stealth heads. You don't have to replace J heads if you have already spent the money to make them right. You spend for aftermarket heads if you have stock J heads and you need the power of really good ported J heads.