Ken,
I'll show my ignorance and ask, what exactly is an "Extension rail?" If it is what I suspect, a type of clamp that joins two hex-rods together, would it not add considerable instability to the camera?
I have a B&J 5x7 set up that I wanted to take on my canoe trips, but found that the rail would not fit nicely in my Pelican case. I needed to cut about 1¼" off. I didn't want to destroy the rail so I went with plan "B."
I went to the local industrial supply house and had them cut a piece to fit. It is steel, not the lighter, fancier polished aluminum, but it works just fine. If I recall, the cost was about 60 cents per inch.