Like someone said before take a peice of angle iron and drill two holes so that it will fit over the lugs flat on the drum and cut to fit in your well, then you can measure and mark tire dia. of choice on the angle and measure from that point to and obstructions
If the PCM don't do it try the additve that was mentioned before it's a friction modifier, to make a long story short the fluid is to slick and the tcc clutch chatters, the additive takes care of this problem.