I'd go with an adaptor and attach to the tripod mount as the easiest way.
or if you're not to attached to it, you could cut the fuji back down so that all that's left is where the rails fit, then stick a plate on the top of it (thickness depending on how high you need the lens to be) so you can screw to the bottom of the bronica.
you'll then need to make a plate (2mm card or mdf should be ok) that can attach the bellows to the bronica body... drill out an old body cap and attach it that way?
you'll only need a small bellows I guess, so you can make one out of card or paper, but I've found an old dark bag will give you all the material you'll need to make one. It's best to use the dark bag on the inside, the stiffeners on top of that and then another thinner material on top of that again. I've never had any light leaks.
I'd only worry about the lens once you've built everything, just to make sure you have one long enough to focus at infinity.
how does that sound?