67 twin turbo dart

Need advise, latest from the dyno shop been a problem after problem getting it to tune. Current issue:
Now that it will rev above 3500 however under load it won't rev and even at 15psi of boost it makes no power. Everything in the log looks good, but the motor sounds like it's laboring to get to 5k. It seems as though it may be the transmission but not really sure at this point.

Any thoughts? Trans is trazact stage 4+, PTC turbo convert. But I'm almost thinking boost leak either at a clamp or even the eBay waste gates could be leaking under boost not allowing it to make power. Racking my brain. Help