From the base to the end of the hub is 18.5 mm. But I notice sort of a square guide pin on the hub of the E46 that the E34 doesn't have. Sort of like a keyway. There are also holes for the turn signal cancelling pin and other gizmos that look different. You might have to file off that keyway looking thing on the outside. For some reason, the hub on the E34 is not concentric all the way, there is like a side of it lopped off at about 7 o'clock as you're looking at the wheel.

Hope this helps, good luck Jon. You might be on to something here!

Jr