When I had my grill converted with stones and bricks and such, I had to vary the burner intensity... it was a 3-burner system.
I put the middle to low and the left and right ones to max. I also double stacked bottom stone where the pizza was (put bricks underneath actually)
Lastly, I let it pre-heat for about an hour and was able to get temps around 750 degrees and cook a pizza in 2-3 minutes.
In my setup the heat came from the burners on the left and right side. The thermal mass heated up well and you were able to get nice even browning on top and below in about 2:30.
The distance between the top and bottom was just under 5 inches I think.
The trick was to make sure the middle burner was low so that the bottom didn't heat too high and burn before the top cooked.