The Docter Apo-Germinar 9/300 in Copal 1 is a little smaller and lighter than the Nikkor-M, but uncommon. There was one for sale recently on the Large Format Forum, I don't recall offhand if it's sold.

There's the 9/270 G-Claron, though you might have better luck finding hens' teeth. And a 270mm Computar or Kowa-Graphic, more like dinosaur teeth.

Fuji made two 250s. The f/6.7, with a 398mm image circle, is complete overkill for 4x5. The newer f/6.3 came in two versions, both of which cover a bit more than the minimum for 8x10. I don't know about their weight, all three came in Copal 1 shutters. But then 250 is might close to 240, and you've probably already got one of those.

And Ian raises a very good point about stability...