Almost certainly the ammeter does not have 35A going "through" it but is measuring the voltage across a very low resistance shunt. Now, if you are installing something in your car that is going to require 70A of additional current beyond the current alternator's 35A capability, you could possibly start a fire if some conductor designed to carry only 35A suddenly has to carry 105A. In addition, the shunt your ammeter uses to convert voltage to amps could overheat, just like a fuse. The best way is to replace the shunt by one designed for higher amperage.