I'm not telling you what to do, or not to do, just want to expand on something others have mentioned.
Fuel economy and fuel efficiency are NOT the same, and lugging an engine (especially one with old-school cylinder heads) does NOT usually lead to good MPG.
Many engines (at least those without
adjustable valve timing, swirl-and-tumble cylinder heads, active intake manifolds, ECU-controlled ign timing, etc) want to have some RPM in order to have intake air velocity that leads to good combustion.
So putting an old-school engine with the drivetrain gearing of a modern car (remember to calc with tire dia also) isn't guaranteed to give the same results as what the factory engineers came up with.
T56 gear ratios for a Viper seem to be
(2.66, 1.78, 1.3, 1.00, 0.84, 0.74) - actually not as bad as I expected for 6th.
So, axle gears with 6th OD -
2.76 * .74 = 2.04
3.23 * .74 = 2.39
3.55 * .74 = 2.62
3.73 * .74 = 2.76
3.91 * .74 = 2.89
Lowest axle ratios I have seen from Mopar is 2.20, and I can tell you that is a terrible ratio for anything other than a factory-built malaise-era car.
(80 Diplomat, 318-2 barrel, 904 trans with lockup TC - a very consistent 20mpg highway)
Also, what would be your baseline MPG for a 383 Newport on level ground with 2.76? You should exceed that by eliminating the TC slippage and losses in the AT.
But - how much can you realistically gain - and is it worth it?
How much can you improve MPG with a heavier car, larger engine, less aerodymanics? (vs my Diplomat example, I'm not saying the Diplomat was optimized, it's just a data point to compare)
I would be very surprised if any bigblock C-body, even with the optimum drivetrain combination, could exceed 25 mpg at 60 mph on level ground.
I'm not really interested in burning rubber or stoplight racing. I am interested in how far I can stretch 25 gallons of gas across Kansas.
I doubt you will get good MPG at 60mph with the 2.04.
You might with the 2.39 IF you can drive with a very soft/deliberate foot AND MPG feedback. And your target speed will be important.
I've had a few late 80s cars with MPG readout and can tell you that if you are lugging the engine it is VERY easy to eat into your MPG with poor foot-control on the throttle. You can absent-mindedly give it the slightest increase in throttle with very little results in speed because the drivetrain responds so poorly.
If you are using cruise control, I can assure you that you will get worse MPG the lower that OTGR is. To get max MPG your roadspeed must vary somewhat - you need to slow down a little (to hold throttle) while cresting hills (when possible) and then pick up a few mph on the way down. This can be an annoyance to other drivers, though.
You should also research the factors that reduce MPG to make sure you're hitting the right ones - road speed, aerodynamic drag, weight, rolling resistance (tire tread design, tire width, all your vehicle bearings) also play a role besides engine rpm.
If you plan to drive 55-60mph across Kansas, 3.55 'safer' choice, maybe 3.23 might be OK.
If you plan on driving 70+, you need more torque to push the wind, so look to 3.73 or 3.91.