Sorry to be a scratched record as every time these 645 vs. 6x7 questions come up I suggest the same thing ...

get yerself a RZ67 with one or more 6x7 backs and one or more 645 backs then you have both formats easily interchangable whenever you want - if you have two lenses you really end up having four as the smaller neg effectively lengthens your focal length -

Instead of fussing with the 645 finder masking I have lightly glued in some fishing line along the 645 frames on the viewfinder to get a general idea - It doesn't really affect my framing ability with 6x7 - in fact it helps me get my horizon and verts ...

I use this system and find I use the 6x7 a little more - but I think its due to the fact I have two 6x7 backs and the lenses i use.

I think there is a 645 back for the RB also although it is harder to find