What's the change in gas mileage when you change your carbeurator?

You won't see much increase in mileage if your current carburetor is in good working condition.

When driving a vehicle with a 4 barrel carburetor, you are mostly driving with 2 primary barrels. These are almost always smaller than the barrels in a 2 barrel carburetor. Then more power is needed, like in an on-ramp, passing a car, towing, etc, the secondary barrels open as necessary to give the engine more fuel and air. The secondaries don't even open all the way unless you are under a very heave load such as hard acceleration or towing a heave load.

If you drive using a little restraint and use the 4 barrel only when needed, you will get better fuel economy with the 4 barrel than the 2 barrel.

The only thing I would consider replacing a 4 barrel with would be a throttle body fuel injection unit. But this may be a little costly for a 30 year old vehicle.

If you keep your foot out of it, you'll be fine. As the previous posted said, a 4bbl only uses the front two barrels for normal driving. At approx 1/2-3/4 throttle, the back barrels start to open. 15 mpg in an older 4x4 is doing pretty well regardless.

This is a hard question to guess on. With any carb. if adjusted correctly, you will get the performance (or lack of) that you desire. An other way of reducing power is to put a restrictor plate under the carb. Any way you do it, it will require some tuning.

Tornados are snake oil and have been proven over and over again to be useless. Keep the 4-bbl and re-jet it. Take it to a carb shop and have them do it.

originally that engine came with two barrel carb..

if I wre in Your place I will leave it with 4 barrels.

buy a tornado for the air cleaner they work great.