I guess this could go back and forth but in reality your MPGS are obviously not accurate ,with 2.73's I don't average nearly 26 miles per gallon, and I do primarily highway driving at conservative speeds. I average about 23, meaning 410's aren't going to touch 20, not in normal driving conditions.
I can't speak about automatic transmissions, but my friend has 4.10 gears in his manual car and it's horrible for street driving, 1st gear is basically useless unless you don't mind shifting immediately, and even starting in second gear his tires spin like mad by just slowly accelerating off the line..
I would think that without doing other mods to your car, like wheels/tires, LCA's etc, 4.10's just wouldn't be practical. If you call the local Steeda shop here in Pompano Beach they won't even install 4.10's they work only with FRRP 3.73's because they say anything more than that is a waste unless you mod the car to be able to handle it.
Hey, what the hell do I know.
Again, I think you are missing the point. 50 miles highway. HIGHWAY. We are talking only highway mileage in his situation. I get 26mpg on the highway with my 4.10s, and that is a FACT. I get 17 in the city.
You missed the point on the second part too - he has an automatic, as do I. I don't care about 1st - my car shifts for me. In fact, it's nice to lurch out of 1st into 2nd when I want to, at 7000+ RPM every time, without having to watch a gauge or listen very carefully.
Modding the car to handle 4.10s is as simple as a tune, LSD, and driveshaft + safety loops, most of which he was already considering.
I don't know why you are so against 4.10s, but it sounds very illogical, especially given the OP's position.
Edit: That sounded more hostile than I wanted - I simply wanted to illustrate that he is obviously looking for the most fun with a minor budget. 4.10s will get him there with only a very minor total cost increase over 3.73s. We aren't talking 4.10s vs. 2.73s here - he's already made the decision to get gears (pretty much).
I say go for it - get gears, tune, driveshaft, LSD, exhaust, suspension package - in that order. Most bang for your buck for the most part. Just go easy on the foot in 1st before you get an LSD with those 19" wheels.