Chevy under-carbed the six through the years. Using larger (or more) carburetors moves the power curves higher on the rpm scale. You will lose some of the grunt at low rpm, but you will also have more power at the top end.
How much carb is needed depends on what else you have done to the engine. What goes in must go out, so a split exhaust of some sort is helpful with dual carbs. Also, a camshaft with more valve lift and duration is another plus. Neither of these is mandatory, but they help to use the added airflow. If your engine is otherwise stock, a pair of the YFs would be a good start. If you have the split exhaust and cam, consider dual Zeniths or even triple YFs.
I doubt if you will see an increase in gas milage, unless you have a horribly inefficent carburetor. Tuning multiples isn't that difficult, but they do need to be matched with the proper size jets. It is the jetting that determines your air/fuel ratio.
Another option that will increase milage and performance is an HEI distributor.
For more info on carbs, search this forum for posts by Jon, aka carbking, bolter #2929. He rebuilds antique carbs for a living.
