Pack film Speed Graphic.
Since FP100C is 3.25x4.25, I've been thinking about how I could adapt a polaroid back on a 3.25x4.25 Speed Graphic. Perhaps something like a series D.
And then I thought about my Hasselblad polaroid back. Could I theoretically take the 6x6 mask out of a Hasselblad back therefore exposing the whole frame?
Would it fit on a Speed Graphic, or could you somehow adapt the back of the SG to accept the polaroid back?
There are Pack film backs that with the Speed Graphic (assuming you have a Graflok back). I think both Fuji and Polaroid made one for 4x3 pack film, and they both work with FP100C. Fuji's version is called "PA-145" and Polaroid's is called "Polaroid 405". I hear that they're both kind of tricky to get a hold of however.
Or in June you can order 3 1/4" X 4 1/4" film from Ilford.
Here someone hacked a Hasselblad Pola back onto a 2X3 Century:
Lots of cheap Hass. Pola backs on eBay. Please do a post if you can make it work on your 3X4.
Oh, I see now that it was the smaller version of the Speed Graphic, then disregard my post, terribly sorry about that.