How about making the first four prints, if you have a copy stand available hold them flat with a piece of glass and photograph them with your 6x7. You would then have four images on one negative. You could increase the number if you were going to use larger paper to print on. That said, photographing a photograph may not give quite the quality as the original. Using a mask as you mentioned would require moving the easel for each exposure. I did that once to print four different images on a single 16x20 sheet. It was very time consuming and cutting a mask on mat board to give sharp borders is a nuisance. I ended up taping metal strips to the paper and covering up the part that was not being exposed.

The easiest way would be to photograph with Ilford BW 400CN and have them printed at a local one hour. It might also be cheaper.

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