I doubt the lights dimming have anything to do with the alternator itself. If the battery is fully charged, it should run for several hours on just the battery, and the lights should be bright with the engine off. Since the lights are brighter when you rev the engine, there are probably poor connections somewhere from the battery. My guess would be the ammeter connections at the dash cluster. That is because all current from the battery is first metered thru the ammeter (except for a few underhood loads like horn, starter relay, and starter). After the ammeter, it gets to the junction where the alternator and all cabin loads attach. Feel the connections since a voltage drop would make them warm. Measuring drops with a multimeter would tell you much more. You should also check the battery terminals, although if corroded, you would probably have trouble starting.
In my 65 Newport 383, I got 18 mpg hwy with the original Rochester 2 bbl carburetor. I recall even with points, but switched to Crane XR700 ignition early on. In 1996, I put on a Holley Pro-jection (2 injector TBI). I paid too much (new). I didn't notice a mileage improvement, even after I added the O2 feedback. I also tried the later 2Di "digital" box. It seems a bit worse in operation, though the O2 sensor connects easier. With the original analog box, I rarely had to tweak the knobs. With the 2Di box, I have to tweak it constantly, even from cool mornings to warmer afternoons. I can easily tell when it goes lean, and I added an O2 monitor light (which 2Di doesn't have).
I have a Holley Commander 950 which I plan to install once I go back to driving the Newport regularly. It adds more sensors to work better - manifold pressure, inlet air temp. It can also control spark timing if you use a GM 8-pin HEI or similar Ford TFI ignition (cheap at Junkyard). I think the "last gasp" of the Holley Projection (4Di or such) was similar to the Commander.
Re cost, I got a complete Holley Pro-jection off ebay for $180 a few years ago. I got an ECU box for $15 and missed a TBI for $20 near me, and a bag of sensors for ~$15. So buying one in pieces can be cheap. Downside is you might have to make your own harness (not easy), but I don't like their factory design anyway. I just use the parts for spares. I got a Commander 950 system for $600 on craigslist (list price ~$1800). I also got an ECU alone for $120 on ebay. I will have to make a harness, but I found the ECU connector is the same as GM 90's computers ($15 used) and I will use a harness from a Magnum engine since I plan MPI for that anyway. You will have some issues running an EFI fuel pump. See my post on forabodiesonly (same name) and others. I did that on my Newport and Dart. Actually better on the Dart (one learns).
If you prefer to run a carb, the Carter Thermoquad or Rochester Quadrajet should be best. The problem is good ones are getting rare, as are parts. I have one of each I plan to try. I have a unique Offenhauser "Dual Port" intake for my 383 that I will try later.
Many plans, but for now focused on restoring my 65 Dart and 64 Valiant. Actually not even that since putting an engine in a Mercedes 300D. Sitting on a lot of parts, but I have many cars, and I can easily re-sell whatever I don't use. Old stuff usually goes up in value. At least my TQ and QJ should.
