360 Cam suggestions.

It'll get less mileage with a bigger cam, manufacture dependent, it'll be worse or better. Not all cams that have a 270 duration are the same @ .050. High lift has nothing to with mileage loss or power loss unless it exceeds the heads flow capcity.

What car, it's weight, carb, tranny, rear gears, tire size and any mods?

The cam you chose makes good torque and power at a low RPM. It will be fine for a everyday driver.