The shunt we sell is marked 40A/40mV in order to show you that it is a 1mv/A shunt. It is rated for 60A.
If you do use another shunt, the most we can read is 100mV, or 100A. 300mV should not damage anything, but it will not read correctly.
However, we do not recommend the shunt be in the path of the starter motor, which is the only reason you would see 300A in most planes.