how can it run using less gas

Big block and gas mileage don't mix. ....but there never was a big block G-platform monte, and I bet yours is stock specs. It's probably the 305. You could go to a two barrel carb, you most likely have a quadrajet

I suggest converting it to fuel injection if it isn't already set up for it. That makes things like tuning and diagnostics easier to do as well.

if it is carberated you could try changing jets , make sure the timing is set up properly or you could look at other options like which transmission it has or the rear gear ratio . there are other things but without knowing the car its hard to elaberate.

You could also try using Amsoil. It is a full synthetic and while it won't make a huge difference, I get 19 in the city and over 30 on the highway. I know mine is more fuel efficient anyway but.. worth the try I think GL