Using different sized cranks is not a good idea, because the pedal circumference will be smaller with the shorter crank. (the reach will be the same on both sides, but one foot will come up higher than the other on the upstroke)

I have a 1cm discrepancy and I had a machinist weld a spacer to a spd cleat. I also had a 1cm lift built into the shoe's sole, so the cleat is still recessed and I can walk around normally without the cleat hitting the ground. Works great, sorry for the fuzzy pic