Wait a second. Looking at the sidewall aspect ratio in Gasman's tire tech link, the height of the sidewall in the 120 60 tire will be .6 times 120 or 72mm. The height of the 120 70 tire will be .7 times 120 or 84mm. So the 120 70 tire will be 12mm higher, which raises your triple clamp 12mm. To lower the triple clamp 12mm, you have to raise the fork legs 12mm or about 1/2 inch higher in the triple clamp.
That seems like a lot to me. I would measure the height of your axle off the ground with your stock 60 tire and then measure it with the 70 tire. The difference is what I would raise the fork tubes.