i have a 2007 chevy silverado with 6ol gas engine with allison trans 3.73 rear i would like to change it to 4.10for better towing

When you change to a lower gear it will help your towing but kill your fuel mileage. If it is 4x4 you will have to do front and rear.

When you say "for better towing" do you mean for higher towing loads? The 4.10 rear end will give you better traction and pulling stability but the gas mileage will drop about 8-12%

It will put less stress on the engine and transmission but your gas mileage could drop quite a bit. The way to compensate is to install a superchip and cold air intake which will raise the fuel mileage and give it more power too. Usually 3-5mpg.

will that increase my weight at all so i can hall a heavier load or would that just make it easier to pull?

Just easier to pull. You can add an overload spring to it for heavier hauling and their not that expensive or hard to install.