A 70V spike is usually just that, a very short term condition that doesn't really carry a lot of power, but the voltage will do the damage. Your circuit sounds like it will take care of momentary spikes pretty well.
Now if you are talking complete alternator regulator failure and 70V over voltage conditions, you might have worries. The way this is usually handled is to have an SCR Crowbar circuit and a fuse. The over-voltage trips the SCR into conduction which shorts the power input to the device you are trying to protect, eliminating voltage and the fuse blows to remove the power into the circuit.