20's???
Don't do it even with the right offsets, just wrong and as mentioned will mess up the geometry. 19's are stretching things pretty far as well, but I've seen them used just wouldn't recommend them, either.
The Z32 has a pretty narrow range of wheels/tires that can be used compared to most cars unfortunately, but is mostly due to the suspension. See our wheel size sticky for the best sizes and tire combos to consider, and realize anything outside of those sizes will definitely require work to make them fit, and will ruin the value of your car.