The only real advantage of the 645 back is that it gives you 16 exposures, aside from that, as stated, if you want to compose for a rectangle you can perfectly well do it in the 6x6 frame.
If you want to use the 645 back for vertical shots, a 90 degree prism or a sports finder is almost a necessity. The WLF or the 45 degree prisms are awkward to use with the camera on its side.
If you can't find a mask (likely) you can make one, or use the grided screen, which will show the 645 coverage area.
I prefer the grid screen, since it's always there, plus it's helpful for lining up vertical or horizontal lines in your shots.