If the bellows are from a 35mm system then I am afraid that they will obstruct the image path quite badly. Take careful measurements, tinker around with them etc. Works for me, I gather the stuff I will be needing and just play with them, try to visualise how to fit them together.
Basically what I did is described further here: http://www.apug.org/forums/forum147/...il-camera.html
I am making a 4x5 extension back for it someday. Monorails are fun for portraiture, studio and still life but my heart belongs to folding bed cameras.
I think it cost me some 80€ to make, half of it went on tools.