I don't know about using the original rubber mounts, I've never used them. I have Keihin 41mm, with fuel resistant hose to 'stock' intake manifolds. The intake manifolds and heads have been heavily ported and gasket matched. Doing all the port work , as well as the 'built' motor, required bigger carbs to maximize optimum power/torque. Is your motor stock? If so, you should probably stick to the original carb size.