Is my 340 Overcarbed? Avg 5-6 MPG

A 770 ultra HP is a vacuum secondary carb, correct? List #86770? You mentioned a DP earlier. If it's not a Vac carb, which list number is the carb?

Cool that the idle and crispness are present, that's the first step.

Idle looks good as long as that's the point of highest vacuum. If it has a higher vacuum at a slightly lower idle number, that's OK. Set it there.

Try turning the float adjuster 1 flat clockwise on both ends and see if the low end numbers clean up below 2500. You will likely need to readjust your idle mixture screws as well. Do this How far out are the screw now? Once you do this and if it cleans up your low speed cruise, then go to the next step.

Take at least 2 jet sizes out of the primary, maybe three and see what it does. The secondary jetting if a vacuum carb won't come into play unless you are really into it.

There is a lot of power buried in the bad tune... No reason at all that you can't get it, BUT, it will take a little work.