I did some mock up with the apache heads and a stock bore 6.1 I have here and it looks like it will be fine. I bought the heads next question I have is the comfort level using a solid stem 2.18 with beehive springs. I am planning to spin this thing 7300-7600. I am a little concerned. I was thinking of buying the hollow stem 2.18 manley sells and replace the solid stem valves. What's everyone's thoughts?