That's funny... I thought just the opposite.
A shorter lens, projecting a larger image at a given throw, would have a larger field of view thus having LESS problems with vignetting or falloff... or do I have it wrong?

FYI: I will be using this lens for 35mm. I have another lens for 6 cm. (2-1/4 in.) 120 format negatives. Hope to replace that one, some day if finances permit.

I have a somewhat low ceiling in my darkroom. I can't get the enlarger all the way up to the top of the track without bumping the ceiling. The short, 40 mm. lens is supposed to help with that. IF everything pans out, I hope to be able to make some nice 11x14 or 16x20 prints with it.

I WILL watch out for the problems you mention. I still have the 50 mm. I can still use it. It's just showing its age.
If the 40 mm. doesn't work I can eBay it, I guess, and use the $$ to get another.

I think I'm going to stick with the flat lens board. If I can find a recessed one at a good price I'll consider it.