A 75mm lens will cover 6x6, a 50mm will not. The condenser lenses may need to be moved/rotated/reconfigured to handle a different focal length (at least, such is the case on my Saunders 670). In my Saunders, El-Nikkor 50mm and 75mm lenses thread in with no problem. Most (but not all) lenses are a 39mm thread. And, many, but not all, lens boards are tapped (threaded) for those lenses to fit. If not, you can probably cut your own lens board and use a threaded ring (jam nut) to hold the lens to it.

Dan