if you've already mounted it 'tireless' and found out the bolt pattern is the same, isn't it obvious that it will clear the rotor? if it wouldn't clear the rotor, it wouldn't have fit on so you could check the bolt pattern... or did i read that wrong.

the bigger concern is that it sounds like you want to run one 18" rear wheel and one 19" rear wheel (in a situation where you had a flat). i would highly discourage this! running your rear wheels with different diameters will toast your differential.