340 Gets Terrible Gas Mileage

The TQ suggestion is a great idea, best performance 4 bbl carb there is. NGK plugs would also be great.

But the fact is that with the info provided & parts being used, you should be getting better mileage.
Three things to check as a starting point, relatively easily:
- float level. Should be 7/16". Unlike brand H [ & clones ], you do not need new gaskets if you are careful.
- check that jets & met rods are correct for that carb
- carb reassembled. Loosen met rod covers & swivel slightly so that the pistons are in view but captured; tighten screws. Start engine, put in gear, & watch pistons. They should go down & not move; if not, piston springs are too strong & you get poor mileage.