I would use an 18 on the CP, so keeping it standard. That way the gearing will be correct and the same as the BA with a 19.
Can I suggest putting a wanted advert in Cheval for a postwar BA box? Alternatively, you might get one at the Annual Rally.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.