The clearance between the disk rotor and hub is different on the front and rear wheel conversions kits.
The front wheel has 12.5mm clearance from the inside edge of the disk to the face of the hub.
The front wheel has more at 18.5mm.
A spacer is included with most conversion kits that adds an extra 2.5mm, however this will move the disk closer to the fork/frame and may cause clearance issues with the disk rotor.
It is important to check these widths if you are planning to run disk brakes with your kit.