Thanks everyone for the comments. It seems that maybe the best thing I could do is to look for an existing and working combination. That would save me the hassle of finding the right softboxes and adapters. Might take some time, but as we say here, patience is a thing of beauty.
Somewhere I think I read that the DeVere 504 can rotate the negative. Is that right? And does it work also with strips of, say 4 negatives? It would be very convenient as I mostly do 4,5x6 and I always have to bend my neck in an uncomfortable position to see if the horizon is level when doing landscapes on this format.