What offset is needed to clear 18 and 20 inch wheels on a 1960 impala?

Zero offset. I have a 1960 impala with 20x8.5 on the rear with 245/35/20's and the front are 18x8 with 245/40/18

If the car is lowered you will need to drop the shock mount to let the diff drop down to be able to get the wheel under the arch if doing it on the ground.