Leon and Brian,

Check the lens range for the 6x17 backs on 4x5. I have significant doubts that they will cover the 72-210mm that Brian mentions. I would think that mechanical vignetting will kick in around 150mm, and I'd think that you'd need a deep recessed lens board to use a 72mm. 72mm is more likely than 210mm, I would think, but I'd also think it's going to be limited to centred, straight-on use.