Rear End Questions

[My7d1]

I just bought a 72 Blazer 4x4 with a stock 350, a 4speed and the 3.73 rear. I have a rebuilt 2.73 rear that was going to go in my 71 C-10 with a 250 but with that 350 I am thinking the 2.73 will be way better on gas mileage, my questions are:

1. Will the 2.73 affect the torque?
2. I assume it will so the pick up at a light will be slower, noticeably? Not looking to race just keep up with traffic, get off road in the mountains and in snow (which is going to require highway driving) AND get decent gas mileage.
3. Anything I should be cautious of doing the swap?
4. Assume something will need to be done with speedometer?

Appreciate any input/advice offered.