Fuji 100c pack film is a neg and a positive, its just not marketed as such. Dry out the neg side after developing the print, tape the neg to a piece of glass, and use a nylon brush to paint bleach on the base side (not emulsion side). The black crap will slough off, leaving a clear, thick base that looks a lot like the 4x5 160c base. The bleach clears it.
@TMARK. Interesting! I am new to the Mamiya 4x5 stuffs, appreciate if you could be a bit elaborative? I am incurably interested. (darkroom processing is what I left years ago, or maybe I am dumb! ;-) Thanks!
UPDATE: I found this link: http://new55project.blogspot.com/2010/04/blog-post_6525.html
Really comprehensive. Thank you TMARK for hinting the solution and Bob Crowly deserves also thanks for the innovative instructions.