The Edelbrock carb is a pretty good carburetor. (Got 3 of them myself with 2 currently in use)
My hot rod Ford 302 has dual Edelbrock 600s (CFM overkill I know, but it looks cool) and it gets 20-21 on the highway, which stunned me the first time I checked it. I was praying to the automotive gods for 12 MPG when I first got it running.
You might try decoding the rear axle tag and make sure that you don't have a set of real low gears in it. (4:10s, etc.) With a 3 speed auto a ratio like that would really have that engine winding up.
This one is hard to decipher without truck in hand but some possibilities could be:
Timing not advancing as it should?
Weak engine due to wear? (piston rings, valves, etc.)
Partially clogged catalytic converter? (assuming the cat is still on the truck)
If you do your own tinkering I would suggest purchasing a vacuum gauge. This tool is cheap, bone simple to use, and is one of the best tools in the toolbox in my opinion.
Connect it to an intake manifold vacuum port and it can reveal any one of a number of potential causes within a few seconds.