Cruise friendly low 12's with a 340 Dart

Well, don't port the J heads just yet. You can run 12's on unported J heads with a 340. The extra arm of the stroker would need more port volume, but I think it would be workable towards your goal of mileage while still having enuff to power the car into the 12's.

IF you have the coin for tti's, I'd do it.

I like the idea of a spreadbore carb myself, but to each his own. The tiny T-Q primarys (Or QJ's) will help in the mileage diraction. It may be marginal, but if you keep up with all the little things, it will add up.

If you didn't mind the cost of a roller, I'd say go roller for mileage help since it's rolling over the lobe instead of sliding on iol. But the price difference is huge.

For mileage, try a multi spark box. This is the one area I find them superior to the MoPar boxs. Otherwise, theres no difference.

Intakes can be a funny thing in a build like this. For a single plane intake, I'd look no further than the MoPar M-1. The RPM is the dual plane of choice, but doesn't fit a spreadbore unless an adapter plate is used. Adding height. Can your hood line handle it? Spacers are generaly 3/4 inch.