If you don't have any joy with the US suppliers, an adjustable vacuum switch like this should do the job.
http://uk.farnell.com/jsp/search/re...&isGoback=false&isRedirect=false

If the link doesn't work, just search for adjustable vacuum switch on the Farnell Electronics site.

Hope the project is coming on OK, Stuart.

1950 Chevrolet 3100 (occasionally working) truck:https://c2.staticflickr.com/6/5490/30684982201_6dc479d18d_c.jpg
1928 Chevrolet AB National Phaeton: https://c4.staticflickr.com/6/5573/30137792083_42886d30a4_c.jpg
1932 Austin Special Sports: https://c8.staticflickr.com/6/5793/30116567183_1520e67c4d_z.jpg