Depressing Dyno results. Need advise

Gentlemen,
Well I finally got a chance to do a dyno run and was not impressed with the results. I peaked out at 265 hp at 4500 rpm. and 305-315 torque from 2700rpm through 4400 rpm. As soon as I hit 4500 she starts popping and running really lean.
74 duster. The motor is a rebuilt 440, 10.6 to 1, schumacher headers through dual 3in flomaster mufflers. stealth heads, mild cam, dual plane eddy intake, holley 4160 750 with vacuum secondaries. 3.55 rear gears and 4 speed manual.
The guys that were running the dyno told me I needed to get a 750 double pumper and an electric fuel pump and that it should solve the issue. He suggested a demon 750. He also said I needed to run 3/8 fuel line to the tank.
I was using the interactive carb selector on holley's website and it said not to use a mechanical secondary double pumper unless the vehicle is less than 3100 lbs. From what I understand the 74 duster was about 3300 lbs and I have a big block. (lots of aluminum parts though)
I need some more opinions on what parts I need to buy before I go wasting my money on junk I dont need. What would a better carb be? Why are my numbers so low? Please help.