250 Inline 6 is SLOW

[Matt Man]

First off I know this is a low power engine but this truck is just ridiculously slow now and seems to be getting worse. My 67 GMC C15 has a reman 250 inline 6 with about 7,000 miles on it and is paired with a 3ott and a dana 44 rear end with a 3.54 gear and 26.5 tall tires.

So the issue that I am having is that the truck will barely do 65mph on the highway and the truck has always had a issue with not wanting to rev over 3000rpm. I have verified top dead center and timing, the timing is set at 6 degrees. Spark plugs look good and has been converted to hei ignition. Engine runs smooth and has good throttle response. Another thing is that this truck uses more fuel than my 81 k10 with 35" tires and 4.11 gears! The carburetor has no adjustments on it besides the idle speed.

Like I said, I know this engine is not a powerhouse but I think there should be a little more performance out of this engine. Any tips or ideas?