There are AC relays that transmit 65-70A -- dont forget to increase the gauge of the wire.
These are found in lincolns and trucks as a Glow plug relay or various.
there are enough 50A to 70A applications that you should be able to find something that will work.
http://www.omega-usa.com/client/imag...y_Switches.pdf
Remember , anytime you are moving a buck-load of amperage you are creating heating elements at the same time.
be careful and make sure you have protection for the circuit.
better to have an temporarily overheated car than a wiring fire/meltdown.
Steve