I don't think its the bearing that moves things in or out. If it's the stock axle in the stock housing, then the wheel will be in the same place regardless of bearing won't it?

I have a similar doctordiff kit. My wheel moved out some because I changed to BBP axles (in a stock 8 3/4" A body housing) that use the B body brake offset.

The calipers are Mustang Cobra's - the ones doctordiff included were brand new from Ford Racing - the equivalent of Mopar Performance I believe.

The wheels are moved out a very small amount. A brake drum is about .125 thick while a rotor is about .250 thick at the flange. So that moves the wheel out about .125 which shouldn't make any difference.

DoctorDiff's kits are designed so that the caliper bracket is the same thickness as the drum backing plate so everything goes back together properly. Some aftermarket kits are all screwed up in that area.

This picture without the rotor might help.