Hi Eiger, I had this same problem when I built my D7 framed trials bike.
I found that a Triumph Tiger Cub swing-arm that I had in the shed, fitted straight onto the D7 frame, and had room for the 400-18 trials tyre.
I don't know if all Tiger Cub arms are the same though - so best to double check it if you find one.
Good luck
Baz