Fuel issue?

[mtnman210]

for those that don't know the truck in question, it's my trial rig with the 516 caddy. It idles fine and I can stand on the throttle in 1st and second gear. But third gear the truck just falls on it's face. It stumbles and sputters like it's going to die. I started to notice it was sputtering in second around 4000rpm when I was climbing hills. I'm running a quadrajet carb on it.

I'm gonna replace the fuel filters both the inline and at the carb. I was even thinking of doing a pressure check. Could my fuel pump(mechanical) be going out? Anyone know what my pressure should be? Is there anything else that could be a problem area?