The 290 hp 340 had biggeer ports and 2.02 valves not the 1.88 you have in your heads. Also the single plane you are useing has VERY small diameter runners that are very long and therefore it will be a low rpm manifold not high rpm like a typical single plane. I think the wheel HP you are makeing is right in line with what you have. Headers are the only improvement you have over a 340 everything else you mention is costing you hp when compared to a 340