My guess is it was dropped due to cost considerations. The 3.9 V-6 can be produced with the same tooling as the 318 V-8. It uses the same transmission and exhaust pipes (in the Dakota) as the V-8. The accessory equipment for the V-6 are the same as the V8, too.
As for emissions, Mopar could have tweaked the head and installed Sequential Multi-port Fuel Injection and solved those problems if the engine really had a future for the company.
The Feather Duster and Dart Lite in particular exceed the CAFE requirement and wouldn't need offsetting sales of little FWD cars shaped like cough drops.
FWIW: I'm getting about 14-15 mpg in town and 22-25 mpg highway depending on speed and temperature. The car is a 73 Dart Custom, 2.93:1, 904, 205/60-15 tires. Only significant mod to car that would affect mileage has been the installation of an oversize exhaust manifold with no carb heat, and a 2ΒΌ exhaust.
