Rather then the Bi you might look at the current GT. Like the Bi it hase base and center movements but they make the GT yaw free. The Bi is not. The GT takes lenses down to 65mm on flat noards and, unlike the Bi, you can use film horizontally with a rear center tilt or a Polaroid back and still operate the holder or the back. On the Bi you would need the Polaroid Spacer to do that. The rails for the B are no longer available. The GT has a telescoping monorail, longer ones are available, and the GT does not require a rail clamp so it has a continuous focus track 19" long.

All accessories that fit to the front or rear standard on a GT are the same as the ones for the B so boards and accessories are plentiful. Lastly the GT is current and a new one has a 5 year warranty. There are no parts left at the factory for the B rail or the items that attach to the rail.

The only function the B had that the GT does not have is geared shift. On the GT you slide the standard for shift. The current GTL has geared rise and shift.