Looking for overdrive trani advice for /6, please help

If maximum economy is the goal, emulating the manual transmission OD Feather Duster should be the jumping off point. Finding a stick shift parts car to allow you to convert your existing car to the manual overdrive A833 will probably do more for the economy than any other single thing, followed by using anything possible to lighten the overall weight. A Mustang T-5 five speed (2.95 TO 3.35 low gear) with an S10 tail grafted on would minimize any floor pan mods and be even more efficient than the 833. The Feather Duster components would be a good start (hood, deck lid, aluminum bumper reinforcements, aluminum case 4 speed, aluminum intake manifold, etc.) on dropping weight (187 pounds by factory estimate). With or without the Feather bits, use as much fiberglass as you can afford (hood, fenders, doors, deck lid, bumpers, even Lexan fabbed up instead of glass for windows), and loose the steel bumper reinforcements to get the overall weight down. Finding an aluminum block to build would save even more (good luck with that one).

If you're absolutely set on an automatic, a late lock up converter slant six transmission is probably your best automatic bet, unless you're skilled enough to take on the requisite floor pan/torsion bar crossmember mods needed, and have a butt load of cash (1500 to 2000 grand is probably a reasonable estimate, about the same as a Gear Venders add on unit) for building and adapting any kind of late OD automatic to the slant. I had a lockup automatic from a 79 D100 van until someone wanted it more than me and appropriated it, so they're out there.

Food for thought, Mopar used a 2.7 rear gear in the automatic Feathers, and a 2.9 for the OD manuals.

I would spend my efforts on making the slant itself more efficient. If you're doing this in school, maybe the resources are there to investigate modifying the slant six cylinder head combustion chambers into closed chamber heart shaped ones (like the 302 small block head) and use pistons with a high enough crown to zero deck it to get some quench. You might want to consider running the slightly longer 198 length rods to use the zero deck pistons available for them. As long as you can control detonation, the higher you can run the compression ratio, the more economical it will be. Of course, raising the compression high enough to need premium fuel is probably contra productive to reducing fuel costs. Using bigger valves in the head and/or porting, and a camshaft larger than stock would increase power output, but almost certainly cost fuel economy.

A staged Weber two barrel carburetor adapted to an aluminum super six manifold might work for induction, and a GM 4 pin HEI module adapted to the stock (rebuilt and recurved) mag pulse electronic ignition distributor has long been an accepted mod to make the ignition more efficient over on .org.

If your skill levels and resources are up to it, you might try a lightweight long ram (something up to 20") fuel injection manifold fabricated from tubing and having weld in bungs for port injectors. A low cost way into fuel injection might be to use the components from a Ford 3.8 including the EEC IV ECM and grafting it's on distributor or DIS. Various junkyard EFI components matched to a Megasquirt is another way to skin the same cat. Building a set of long primary headers (30"-32") with long collectors (16"-20") and a 2 1/2" pipe and muffler(s) would optimize the exhaust.

Above all before you turn a wrench, devour everything you can on FABO and over at slantsix.org. There's tons of expertise and experience on these boards and guys here don't mind sharing.

Get a plan, get some help, and get started.