low fuel pressure

Is it the later pump with the Allen Key for the spring or the old pump with a 5/8 socket size?
When you fitted the new Pump assuming its the Allen key type did you buy a spring kit also as the new pumps come with the old springs?

 

Then that will make a difference as the spring is longer

 

here u go.

 

This is where you need a Fuel gauge in the Cab to see if its maintaining fuel pressure during a long hard Pull!!

 

If it's the Allen style pump you probably won't be able to shim it, from what I know installing the correct spring is the way to bump the pressure in this style of pump. The old style are the shimmable kind.