Thanks folks. I was initially inclined to agree with Tim and Andreas, that an evenly illuminated 6x7 neg could be replaced with an evenly illuminated 35mm neg and the lense changed. But now I'm not so sure. I had a look last night and there does seem to be some slight fall off using the 50mm lens. However, it is a very small, cheap, plasticy looking Besler 50mm f3.5 lens. So it may be the lense. Anyway, the column is pretty tall so I think I can print to about 16" with the 80mm rodenstock, so this is about as big as I'd go with 35mm anyway.
Any ideas on where I might pick up the last condenser?