Ok, don't know what you have but am suspecting if its a 750 it is a double pumper or a mechanical advance secondaries. What you are thinking about when you say "2 barrels drive the car untill you go up a hill or something that kicks the other 2 barrels in" you are discussing a vacuum advance secondary. In the vac adv seconday units they use a vacuum line that senses the manifold pressure of the motor (load) and opens them up and closes them off accordingly. With the car off, when you push the pedal to the floor olny the primaries will open and the secondaries will stay closed. On a mechanical secondairy/double pumper (mean the same thing) the secondaries are mechanically staged to open with the primaries. If the car is not running and you push the pedal all the way to the floor, both primary and secondary butterflys will open. Gas milage sucks cause you dump a lot of fuel everytime you hit the throttle, but the engines response to the pedal is much better. One is more for performance, the other is more for effiancy.
To adjust a vac advance sec car you have to change the springsthat govern the vacuum assembly, acclerator squirters, and acclerator pump.
On mechanical advance carbs you change the cam, springs, acclerator squirters, and acclerator pump.
Remember though, performance and gas milage are opposite concepts!!!!
Don't know about dropping the float levels and more high end power. Seems to me that you should set them and thats that. Anyone else know??