Automobiles are engineering compromises. Seeking to "hit that sweet spot", but not always getting there. Desired "styling" is not always the most aerodynamic way to do things, too! "Road hugging weight" can be good on the open road, but not good in city traffic. Adding 50 horsepower (at 5000rpm) is not going to make a big lot of difference in performance at 30mph, even at WOT.
The question is where does it "fall flat"?? The stock 2.7 rear axle is great for open road cruising at 60mph+, plus contributing to good fuel economy at those speeds, too. That "good fuel economy" CAN depend upon the items used to replace the ELB system, by observation. Basically, which carb and which distributor????
Probably the best upgrade would be to install higher compression ratio pistons, to get the CR back up into the middle 9s, BUT that can compromise the ability of the engine to run on (comparatively) inexpensive 87 Pump Octane fuels. The extra money to buy 91 Pump Octane fuel might not seem too bad by itself, but filling a 20+ gallon fuel tank can make it add up quickly. Then compound that on any kind of road trip and you suddently get into $$$$$ territory. Now, somebody with a car can have different sensitivities to those higher fuel costs than, possibly, somebody who regularly fills up a pickup truck or SUV, but "more" always costs "more" in the end.
Does your car have a lock-up torque converter? Is it still operational, if it had one from the factory?
When the "air" sees your car coming, or one like it, it might seek to get out of the way before it is pushed out of the way. LOL
At 55mph, the engine should be running right at 2000rpm. Not too much rpm, but also low enough that a wilder cam can make it weaker in that speed range compared to the factory cam, which means a bit less throttle response or available torque to move things along. Going to a 3.21 would be 2000rpm at 50mph. Not a whole lot of difference, but possibly enough that throttle response might be increased a bit due to the higher engine speed. But also expect about a 5% fuel economy decrease, too, from Chrysler's own figures, IIRC . Better throtte response can mean less throttle input to increase speed a bit, which can also mean the carb stays out of the "power mixture circuit", too.
The camshaft from the factory is just a bit more than the old 256/260 cam of 1966, but the heads can be slightly better with a few port tweaks and 1.74 exhaust valves. Chrysler did a few engineering tweaks to make the engines "better air pumps", by observation. Plus electronic ignitions and lean-limit carbs for better fuel economy and emissions. AND those tweaks did work, when the cars were new and they WOULD beat similar GM and Ford cars in on-road fuel economy. But with a bit more "touchy" driveability sometimes, as @saforwardlook
has mentioned a few times before.
At this point in time, getting more power and possible economy out of a car like yours might take a Holley (or similar) EFI unit that can also control the engine timing. With any EFI, driving style can make more difference in on-road fuel economy because with a carb, as the throttle opens, intake manifold flow must PULL fuel out of the float bowl for the engine, whereas with EFI, the millisecond the throttle moves, more fuel is added to the air flow, which means better throttle response, but can also decrease fuel economy without a steady foot on the accel pedal, by observation.
The EFI will provide a more vapor-like fuel mixture to the intake manifold, which can increase fuel economy on the road. BUT . . . using that as a reason to go to EFI is not a financially-viable reason to do so, by its self. Not sure if you might desire to spend approx $4K to get the EFI kit and related equipment/hardware (and installation labor) to make it work as it should.
And EFI and higher compression pistons can make the expenditures more than the car is worth, I suspect, so little return on investment, other than how things feel when the throttle is moved. Similar, too, if 3.21 gears are put into the mix.
To me, I enjoy the "long legs" of a 2.71 (and tall tires) on Texas roads, where normal Interstate cruising speeds are in the 80mph range (actual rather than posted). At those speeds, the engines are just barely getting close to 2800 rpm (in the meat of the torque curve) and everything works well. And this can happen for HOURS on end, so the quieter engine and better fuel economy of that 2.71 equipment works well down here. Or into NM and AZ, or even in KS. "Trips" in your region might be different, which I understand.
So, unfortunately, no simple or easy answers, but a consideration of priorities and what each of them might ultimately cost. "Change" cost and "resulting daily" costs.
If we know which carb and distributor were used to de-ELB the motor, we might know better what might be done. Plus where the "falls flat" comes into play.