Although there are some very practical suggestions in this thread, I see valid reasons for keeping the 2.73 rear and ditching the 700. The THM 350 is cheaper to overhaul than the 700. The 700 often requires a number of upgrades just to overcome original design issues without even considering what it needs to survive a high torque environment. The 350 has less rotating mass which has the potential to save fuel in town. THM 350's are not popular and when they show up Craigslist in this area they can be had for under $100. The THM 350 is more likely to survive with a weak cooler and is less likely to fail if the detent cable is misadjusted. And although the 350 only has a 2.52:1 first gear ratio vs the 3.06 of the 700, the engine is not required to have as wide a torque curve to provide steady acceleration during the 50% drop in rpm at the 1-2 shift.
I agree with roger55. The question about the final drive ratio really depends on the torque curve of the engine. If you can put cruise rpm at the low to mid range of the torque curve you generally get better overall fuel economy. The Camaros and Corvettes equipped with TPI, six speeds, and numerically lower rear gears achieved very good economy and still had reasonable acceleration despite having mild power numbers. Tuned port engines had peak torque ratings of at least 330 ft/lbs with a broad torque curve providing at least 300 ft/lbs from roughly 1500 to 3500 rpm, plenty of torque for cruising.
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... but longevity of 700s is certainly beyond reproach.
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Although your transmission is indeed remarkable, I will have to respectfully disagree with the conclusion. 30 years as a professional technician, with the majority of my time spent with GM vehicles, has brought me far too many prematurely failed 700R4 transmissions. The list of factory changes and "upgrades," most of which were designed to keep the transmissions from failing while in warranty, is huge. And the aftermarket has thrown no small amount of time and money into the fight as well. Yes, a 700R4 built today has a much better chance of surviving than an old factory unit. It would be nice to attribute this to excellent engineering, but truthfully it is because the aftermarket and GM combined have 36 years worth of trial and error trying to turn it into a decent transmission.