340 Chugging instead of Reving

OK no you missed the point; if your engine begins to run stupid at the crossover point... maybe the box is bad.
I suggest to put a timing lite on it and see what happens to the timing as the rpm goes thru there.

However, seeing the plugs now, wow, they are all over the place.
looks like a couple are oil-fouled, a couple are rich fouled two are a lil lean not much, and two could be called typical; so that's quite a story.

Jus asking; why are you running 72s front and rear yet still with a PV.