Robert's questions are good even though it seems like you may have answered them.
Is 6 psi at WOT all the way to 3100 in 3rd or 4 gear?
Fuel consumption under load is much higher than in N. I'd expect 3-5 psi down the track. That would indicate the fuel inlet valves are open. Your description sounds like it won't go past 3100 in 1st or 2nd. How about on the highway (Interstate)? Will it go to 3200-3300 cruising at 70-75 mph or whatever your gearing works to?

Honestly, if its just hitting like a wall like a rev limiter then I would not suspect fuel. I've made it through the lights with fuel pressure at zero (there was enough in the bowls to just make it although it sure wasn't best time) and I've run on 2 barrels (out of 4) to 6000 rpm in 3rd on the dyno. It reduces acceleration but doesn't kill it or even close.