i see what you mean, and i was going to do it that way, but i was told there was a risk of the nut splitting if i did this (being old brass) - the amount of backlash is significant... it might need quite a gap!

What I had planned to do is saw it with a fine saw, then reassemble and check the backlash. then mill a small amount off the fixed portion such that when the screws are done up its a little overtight.. then shim it back. Its only until I can make the new nuts and screws so only has to last another, ummm, year or so... these have lasted 100y already!