My 340 getting 6 mpg

I thought it was 12 and 34.
The carb isn't right yet, may be the wrong carb 'not size' for the aspirations of decent mpg, but it still needs to be jetted down and driven hwy speeds..reducing the primary jet size until it surges, then go back up 2 jet sizes. Next dial in the secondary jets within 9 sizes of the primary, check vacuum and install pv at 3 to 4" below idle 'close enough for now' then the one of the biggest influencers... the distributor. It's a pro billet msd non vac advance 'which would help if it had one if mpg was a concern'.. it needs a sdvance bushing made to eyelid a mere 8 to 10 degrees so that he can have a higher initial timing.. like 24 degrees 'wherever the starter kicks it back warmed up, then back 2 degress'... and now the carb throttle position can be lowered.. may or may not be tickling the mains atm... and voila... it could get 12 or 15mpg.
If ya follow those instructions... as I said...you'll fix your mpg issue mostly.

Car rips tires well. It's just the final touch left.

As for 340 mileage...
Best I've gotten with a ld340/750 .528 solid 284 112 in at 105.5 was 15 mpg with no vac advance. 3.73 gear at 60 mph