The only differece between 120 n 220 film is the backing paper. The film itself is exactly the same stuff for both!
The film plane is exactly the same for both 120 n 220 backs... otherwise you would never focus a sharp image if you changed backs. The viewfinder is adjusted to match the focal plane of the film, no matter if it is 120 or 220. They both have to agree agree to a sharp image. If you were to have different focal planes for 120 vs 220 one will always be out of focus when changing backs in a shoot. DaHa?
The preasure plates are the same in 120 n 220 backs as are the springs and tensioners. Parts from one can be used in the other but not the counters or the metering mechanisms.
The real differnce in build are the counters n frame metering mechanisms. The 220 mechanism takes into account winding less, no paper, on the take up spool thus metering the frame length differently, by increasing the roll's diameter on the take up using 120 in a 220 back you may have to do some experimenting to see how the spacings fall.
Run a 120 test roll through the 220 back n mark where the frames align?