5spd - yes, of course more gear means more revs; the relevant question is whether or not the increase is significant. There's a 10% increase in engine speed moving from 3.73's to 4.10's. Let's put that in perspective by talking about how that effectively racks up 'miles' on the engine. A 3.73 geared car will cover 250,000 miles on the same number of revs that a 4.10 geared car will use to cover 225,000 miles. I don't think that difference is gonna be significant in the life of the engine - at least as a practical matter.
As for all the talk about mileage - it's WAY more complicated than simple increasing revs, or my favorite - it's been proven that the difference is only 1 mpg. The fact is that the amount of power required to move the car through the air at a given speed (let's assume flat terrain for a moment) is fixed. It doesn't matter what gear you're in or what gear's in the rear -- the amount of HP required stays constant. The amount of airflow through the engine is not just a function of rpm, it's also a function of throttle opening. Because at a fixed cruise, the engine only has to produce a VERY small fraction of maximum power available at that rpm. At a fixed 70 mph, it probably only takes something on the order of 30-40 HP to keep the car moving. So, let's say the 3.73's are at 2000 rpm at that speed. With the 4.10's you'll be at 2200 rpm. But since the amount of power being produced is the same, the throttle opening will be slightly smaller with the shorter gear. So you can't simply conclude that at higher revs - you'll move more air and that'll take more fuel. That argument would indicate more power is being produced - but it's not; because speed is fixed. And power produced is a function of the cruising speed, not the rpm level. Power AVAILABLE is a function of the rpm level. It's likely that more fuel will be consumed, but the percentage will likely be less than the % of gear increase.
You're gonna have to check if you want to see how fuel mileage is affected by rpm in your car. I did some experiments in my wife's car with the trip computer on extended runs -- running in 4th gear and then 5th gear at different road speeds. Her car has about 15% difference between 4th and 5th gear. That's almost identical to the difference between 3.55's and 4.10's. At 60 mph, it used 5% more fuel; at 65 mph it used 7% more fuel; at 70 mph it used 10% more fuel. Her car is newer and fairly aerodynamic - the differences might be slightly greater in an older Stang.
CleanLX over on corral gained about 2mph over 373's on his practically stock car. Exhaust, gears and a mass air meter were his mods. Car runs high 12's @ 104ish. NO lower than 410 on an aod it will not hurt mileage. Friend has them in his stock 87 aod gt so trust me. Like michael said, it isnt a huge rpm increase, maybe 200-300 and they'll help with speeding tickets on the hwy because you wont be cruising too fast 
Think about it like this, with 273's you have to get on it harder to move it and with more gear you wont have to, so 'in theory' your using less gas to accelerate the car 
410's for your aod car, RsStanG1987. My friend with a h/c/i 306 94gt just did 410's over his 373 and as soon as he got them he kept telling me how much I need them lol. My rear end hums so I need to have that fixed so im pretty sure i'll be doing 410's.
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...273's to 410's is different of course...
