OK I`m in the middle of doing this so here`s what you need to know:
to change to a 120 you need a 3.5 X 21 instead of a 2.15 X 21 Rim
you have to change from a 3 inch fender to a 5.5 inch fender and fender struts need to be changed as well.......I used struts and fender from a Pitbull
Ride and handling big improvement looks killer wilth my springer