Hey Bill. Welcome
I like the Schneider Componon-S or the Rodenstock Rodogon (im unfamiliar with Nikkors for enlarging)
in 50mm for 35mm negs,
in 80mm for 120 formats like 645, 6x6 and even 6x7 if you get one that can cover
and for 4x5 you can go 135mm or 150.
All those except maybe the larger 135s and 150s will fit the 39mm lensboards.
Someone can tell you how to mount the larger lens if any special boards are needed.
Sorry I'm unfamiliar with that Saunders other than using an LPL in a school darkroom in passing.