Hey Pat, the sudco link I posted has carb sizing recommendations on the first page. Sudco does a 35mm FCR Kit for 750/850 Norton Twins, I reckon 35mm FCR would do just fine for a 900 twin, 39mm FCR's could be too big.
What do you think of the 36mm TM MIkuni pumpers?
Have you gone off pumpers because of previous problems with the 42 HSR's?