Thanks for the detailed replies, keep them coming as I progress through this.
I don't think its valve train harmonics. The studs are girdled and pushrods are firm. I am almost convinced that the problem is a reversion. The intake has black soot inside and it goes all the way up to the bottom of the carb. I noticed the black before but never really took too much notice of it. Making changes to the intake system, i.e. bigger or smaller carb, air filter and no air filter, made changes to when the power dip occurres. Will connecting a MAP sensor and EGT confirm this 100%? Does the EGT sensor need to be right at the head to be a true reading, or can it be at the collector? Will also try a 2" spacer to see if the reversion will change significantly. What else do you guys think?
hcompton funny you mention the tyres. I am using 28x12.5 et street radials so yeah they have no problem putting down the power. Well on the track anyway, they do slip a little on the street. But the dyno guy mentioned that a semi racing tyre with a racing compound and side wall is the worse thing to use on the chassis dyno. He said standard street tyres are the best as they don't flex and twist or heat up under load.
So lets say it is plenum reversion, what to do from there? Is it a bad valve job or port job? Wrong cam timing?