OK, time for baby steps here, bucco. Dreaming of exotic transmissions or fuel injection solutions as a way of upping the car's fuel efficiency is fine, but you need to go back to basics.
1. Do a proper compression test followed be a leak down test. That's going to tell you how well the rings and valves are sealing. At this point you don't know if the rings are shot and you need to rebuild the motor, or if doing something as simple as replacing the valve seals will do away with the oil burning problem and let you move on to other areas that need your attention. Until the basic motor is sound, you're wasting your time on the peripherals.
2. When's the last time someone ran the valves? A 74 slant has solid lifters and just running (adjusting) the valves will make the motor run better and be more efficient. It's .010 on the intakes and .020 on the exhausts if memory serves me right. If not, someone here will correct me. Some like to do it static, but I prefer to do mine running. Done correctly, either way will get you there.
3. Go over to slantsix.org and do a search for the HEI conversion thread(s). Converting the ignition to HEI by using a GM 4 pin HEI module conversion is an inexpensive way to up your economy by making the ignition more efficient.
4. If that 4 barrel manifold is unheated, chuck it (at least over in a corner of the garage) and throw the SuperSix manifold on after carefully rebuilding the stock BBD. You may have to play around with the jetting a little to get the mileage up, but once you get it dialed, you shouldn't have to screw around with it. I do hope that when you acquired the SuperSix manifold you got the transmission kick down linkage, because if you didn't you'll need to find it. By the way, don't buy a rebuilt carburetor from the local supply house, they are usually (almost aways) JUNK! Just rebuild your own with a quality kit. If you want to keep your headers you will need to figure out a way of getting heat to the intake manifold, either by routing exhaust like the original manifold or plumbing water from the radiator to it. A simpler solution might be to get rid of the headers and go back to the stock manifold. Splitting the stock manifold to use the rear half that bolts to the intake and buying one of Doug Dutra's front ones is one way of having a fully heated intake (like stock) and keeping dual exhaust
5. Verify the rear end ratio, the Feather Duster used a 2.76 behind the automatics for a reason, and that reason was that Mother Mopar's engineers felt it was the best one to get the best fuel economy with the automatic behind a slant.
6. Jack up that motor and FIX THE OIL LEAKS! Working hard to raise your fuel mileage to save a few bucks is useless if you're blowing those same bucks buying case loads of oil. Might as well replace BOTH motor mounts when the motor is in the air to replace the oil pan gasket while you're at it. Don't want to replace them again anytime soon? There's a guy up in Oregon (
www.polybushings.com) making polyurethane inserts to go into your existing brackets for 34 bucks a side.
7. If you're serious about fuel economy, don't run anything wider than the original size (narrow by today's standards) wheels and tires (195 cross section on a 5" wide rim).
8. Dual exhaust can be really annoying on a six, running a crossover pipe and adding in a second set of small mufflers will do alot to tame it down.
Just a start here and not very glamorous, but you'll probably be amazed what just doing the basics and doing them well will accomplish.
