My Yashica Mat 124G door has a little plate that you slide up for 220 or down for 120. As well, my RZ 6x6 back has a similar feature for 220 film, where you twist and rotate the plate depending on 120 or 220 film. I still have 2 boxes of 220 TXP in my freezer which I love using.

As a side note, is the biggest problem with using 120 film in a 220 back the film flatness or is there something else like spacing I am missing?