Cruise friendly low 12's with a 340 Dart

The cheapest and easiest way to get OD is with a late A-833 4-speed overdrive manual. With a little work they can be almost as strong as the beefy 4-speeds of the muscle car years, and with stock ratios they have a high .73:1 ratio (you can also get a super-beefy OD 4-speed gearset from Passon, but that costs around $1500). Couple that with maybe 3.91 rear gears and it would be very street-friendly while also being quick (BUT it is a manual, that may not be your thing). For that engine you need to update the technology if you want to go fast while being economical, and for that you'll need some good-flowing heads with closed chambers and high compression (Magnums) along with a modern fast-ramp cam, good intake, and good headers. If you can't do that though, I'd say port your J heads (mildly) and get a Lunati VooDoo series, COMP Xtreme Energy series, or Hughes cam, maybe around .500" lift and 268-272* duration. Also get some good headers (others will recommend these) and a crossover for your exhaust (if you don't already have an H-pipe at least). You could also try an Edelbrock RPM dual-plane intake. I'm pretty sure these would get you into the low 13's, possibly the 12's.