I don't know how I can help you here. I can give you the Cat number of the adapters for SB 54's. It depends which SB 54 you have (SB 54 H or SB 54M):
- Adapter for the SB 54 H ---> Sinarback FW/Fuji GX 680 Adapter Kit (Cat 552.65.042)
- Adapter for SB 54 M (or 54 MC, or 75 H) ---> SB 54M-75H/Fuji GX 680 Adapter Kit (Cat 552.45.042)
I hope this helps.
Edited for Addendum: And by the way, WELCOME on LLF!
I was wonder if anybody has any experience using a Fuji GX 680 with a digital back. I've recently aquired a Sinar 54 and know that they offer a adapter for the Fuji. I'd like to be able to use the back on my Fuji. The primary reasons being the tilt and shift capabilities of the Fuji. I know that it's HUGE and clumsy as compared to say a Mamiya 645. Still, I have all the lenses and complete set-up for the 680 and like I said, the T/S ability is important to me shooting interiors.