Chassis at ride height, chassis level (only if chassis will ride level), in other words set chassis exactly how it will ride. Level front to back, raked, or lower in rear. Then carb mounting pad should be level. However a slight ammount out of level won't be that problematic. So unless you are going to drastically change the tire diameters it shouldn't have too ill an affect. Better yet if you know the desired tire heights that you want to run, set the chassis at that height and then level the carb pad.