Quote: If you can't tell, I'm goin' for gas mileage here since my wife will be driving the car quite a bit. [...]
I'm a little scared that I'll regret the lack of power, but since this car will get lots uf use, I hope I'll get over it.
That's a common fear/regret. Either size engine will get you where you're going. (Perhaps it's more important how your wife feels, if she'll be the primary driver? Just a thought...) 
Given your reservations, I suspect that if you choose the smaller engine, you'll wish you'd gone with the larger.
You may come to accept the shortcomings of a 170... meaning plan ahead (far ahead) if you think you're gonna pass anyone on a 2-lane road. When one considers how much more satisfying a 225 is to drive, the extra fuel it drinks is surprisingly small. All other things being equal, using stock camshafts and carbs in stock mid-60s A-body cars, my experience with various engines (steady state highway mpg) is 20-22 with a 318, 22-23 with a 225, and 26-27 with a 170. This is when all are in good condition. When not in good condition, all bets are off. I got 13 mpg highway in a 170 once.
Changing to the 2.76 gears you're considering, a good 225 in an A-body ought to return 25 mpg easily, probably more. Remember that you can overdo the overdrive concept: if you run at a lower rpm than the peak of the torque curve, the engine won't be quite as efficient. I really don't know whether using such gears with a 170 would give you the same percentage increase that you'd see with a 225... that little engine would have to work awfully hard to power such a tall ratio, especially if you ever drive in hilly country. I definitely wouldn't try it without changing the low gear set in the 904 tranny, or you'll be swearing at the car every time the traffic light turns green. (A stock 170-auto-3.23 in a 65 Valiant is barely sufficient-- I got 0-60 times in the 14-15 second range.) If you get some data points with a 2.76 gear, please let me know.
Personally, I love the 170, but I've learned to live with it, and i don't expect it to impersonate a V8. (More importantly, I have a different V8 car for when I periodically need my fix.) Actually, even when warmed up a bit with a hotter cam, it's still no speed demon even compared to the average little Toyota or Honda of today. Depending on your local congestion and the courtesy of other drivers, it can be challenging to weasel your way into traffic...
It is satisfying to watch people's eyebrows rise in surprise when they hear you're getting 27mpg in a 40+ year old car. But most of them would be equally surprised to hear of 25mpg, I'll bet.
- Erik
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It sounds like you've got a cool cruiser/project car, and you've stockpiled some good parts too.