Help on 408 build

What are your gears and do you know when your timing is all in by? Do you know your head gasket thickness or P/N? There are a more than a few who did not set there quench to .035-.040 with the J heads and have had similar problems. What spark plugs are you using? RN12YC Champions or going one to two steps colder may help. No iridium, platinum, unobtanium spark plugs. My first inclination is to thoroughly check for a vacuum leak and to confirm your fuel pressure/float height are properly set before checking the A/F ratio. HRM has some dyno testing with a 408 stroker mopar, they show optimal jetting in the neighborhood of 71 primary 80 secondary with direct to manifold mounting and 73P/84S with a one inch spacer. Typically dyno tuning of jetting is a little on the lean side on an actual vehicle but it's a good starting point.