I have gotten a nice durst 609 and it came with 50mm lensboard (39mm thread) and some other lensboard for 75mm or so with 31mm thread, is there an adapter for it so I can use 39mm lens with it?
I'm not sure about an adapter. I imagine it to be hard to adapt a board to a larger diameter. However, you might want to try mounting your 75mm lens on the 50mm board. It would make sense that the 50mm board is recessed. Even then the enlarger might have enough extension to focus the lens for anything but really small prints.
A normal lensplate (not recessed) with a 39mm thread shouldn't be too hard to find, though.
the lens I have for medium format is 105mm, sort of double the length of normal lens, not sure if I can get an 8x10 out of that, and do not want to do wall projections. I was just wondering if there were some adapters, lenses in the old days were smaller than 39mm, but now I think those could only be found by joining some archeological digs