Cruise friendly low 12's with a 340 Dart

Low 12's and high mileage really do not go together well. This is a bigger problem with small blocks. A tad more so with a short stroke 340/318. Stroking the engine is an excellent soulution to easy power to move the car well, less effort.

I think they best way to go about getting this dual purpose machine into a higher mileage area would be an overdrive trans and a set of 4.30 gears which pan out at about a 2.96 in over drive.
Or 4.56's that feel like 3.14's in overdrive.

The Edelbrock spreadbore carb IMO is a good idea. They have a version of the Quadrajet. It's hard to ignore brand new, but, if your anygood with tinkering, the T-Q is offered in 800 & 850 sizes, are cheap by compare, easy to rebuild and take to a few mods very well. Very tuneable if your willing to sort out rods and jets. I have bent up AFB rods to suite. There 3/4 inch to long for a T-Q, but with the simple bend, the veriaty sores through the roof.

Further info can be had here; http://thermoquads.com/
You tell'em I sent you over from here for help. "Dave" the owner/builder is a good man.

Your goal is not an easy one to do.