TBI won't help economy on the Dart due to the poor distribution of the OEM manifold. The only tested Multi-port systems that have had any substantial testing and use are the GM 3.8 batch systems. Sequential is possible with a crank trigger, but you join the ranks of the experimentals. Any single stage FI system won't improve economy. Applicable single stage EFI systems hold mixture control in a narrower range than a well tuned lean burn carb can do. Done right, a correctly set up carburetor can reliably deliver lean burn in the 17:1 range, which is about the limit of what the metalurgy and combustion chamber technology of the slant parts can tolerate. Fortunately, pistons can be replaced with more advanced designs and materials now. The problem being cost. There is very little that can be done about the head without major expenditures.
The Dart Lite had a very agressive timing curve, and gobs of EGR to avoid detonation. Keep the compression at stock, or lower to avoid problems if your running the OEM system.
Do you still have the original carburetor? They were not the same as the conventional Holley 1945 that was being used on cars during that era. Avoid the later lean burn systems. They were problematic.
The calibration was leaner on the Dart Lite and Feather Carb, and there was more signal to the vacuum amplifer for the EGR. Governer was a 15R in the distributor, and they carried a crazy vacuum advance can.
The guys that are providing reputable economy reports put the 4 speed car at about 34 mpg. Not the advertised 37, though at hypermiling speeds, it might just do it. Getting the full 55° of advance would require traveling at much higher engine speeds than would be legal.
Most of the guys serious about EFI on the slant seem to have taken setting up mega-squirt systems on as a long term hobby. I think Sam has been at it for ten years and is still fiddling around. He runs a T-5 also, but his calibration is aimed at power, not economy.
Most of the T-5 guys built their own parts to mount the tranny up. The only parts available for mating up different transmissions to the slant are for GM automatics. No over the counter parts available I'm afraid. Talk to Sandy about the five speed if your set on going that route. The final drive ratio is still 0.70 so no advantage, though it should be lighter than the O.D.
I'd say stick with the 4spd O.D. for simplicity.
The late cam could be changed out. Effectively, all those cars used the MP244. There are some options that are a bit less lift, and a wee bit less duration. I don't think it would amount to much. Those were the early grinds, and all it would amount to is less power. We still have to get 3600# rolling, and net power was 95 hp even on the normal slant cars. The cam profile technology today is much better than it was 40 and 50 years ago.
The Erson RV10M would be a nice addition. It's profile is closer to the 244, and lift is still reasonable. The lash ramps take advantage of the Mopar lifter size, so tip up and drop out quicker.
Check with DusterIdiot. Even running a modified split plenum Hyperpak manifold with the 390 Holley, he gets up around 26 mpg with the CompCams 252S. With less cam, a more reasonable intake manifold, and a slight nudge in compression, that could be the norm.
Small cross section 4 bbl intake would be the Offy. The Clifford short ram carries too much cross section for low RPM use. Drag racing yes, economy, no.
The new Hurricane can be had in a two barrel version. A 5200 series on that should work well, though in low temperatures heat soak would take a while, so economy wouldn't be great until the plenum floor heats up.
They also cost well over $400. They can be had with injector bungs if your set on spending your time with fuel curves and timing adustments.
I'll be retiring my modified Offy in a couple of years. The Triple DCOE Redline setup probably won't deliver very good mileage, but I only drive the rail a quarter mile at a time.
My daily driver is a 2 litre cargo van.
2¢
CJ