Looking for overdrive trani advice for /6, please help

Dan is right, the place to start is a solid EMS with a feedback loop.

Swap the intake for a 4bbl manifold, and run a 4150 replacement EFI unit from FAST / MSD / Powerjection III, etc. Run that split 3>1 exhaust header, it scavenges well. Upgrade the ignition to a CD system of some sort.

Once you are getting power from every drop of fuel, then you can address the transmission.

There is no reason to replace the 904 with a larger, heavier unit. Go through the 904, and make sure it is working properly.

The big change is the torque converter. Things have improved a lot in 40 years. You need to drive your car and pay attention to the tachometer. Whatever you use for a torque converter needs to stall below the cruising rpm. Newer torque converters can be built to do what you want. Once you know a little about the RPM ranges you operate at, you can call someone at ACC & have them make a converter for your needs.

It might seem backwards, but you may need to raise the stall a little. A better launch from a stoplight burns less fuel if you can stay on the torque peak of the engine. It used to be that a "high stall" converter felt like the car was in neutral until you raised the RPM & it would "bite" This is no longer the case with more modern designs.

I swapped out a dead 5 speed OD trans in my pickup for a C6. With a 4.9L I6, it gets better mileage around town than it did with the manual transmission. I had ACC/ Boss Hog build a needle bearing converter for it, with a 2400 stall speed. I never changed the rear end, so it burns a little more fuel at 75 mph & above. 2 years of mileage logs tell me it has saved me hundreds in fuel costs.

B.