If the film to lens distance is only 70mm then the bellows I was thinking about will be too big. I have a couple of sets from a smaller plate camera which might be useful though. You could make a simple box then put a small bellows at the lens end.
However, if your focal length is around 70mm, you will have a lot of depth of field on 5x4 film and accurate focusing will not be needed. In fact, I think it would work o.k. as a fixed focus camera for most normal subject distances.
EDIT: Actually, the bellows are about 30mm closed so if they are any good for you, you can have them.