MiL:

Tru-Turn makes nut adapter sets that compensate for the short crankshaft/thick prop syndrome. Essentially you order the apropriate adapter and then drill the prop out to the OD of the nut adapter. It then screws into prop and onto the cranskhaft. Very simple, very easy and reasonably priced.

wsmalley:

As noted Fox Mfg. Co. offeres prop hub extensions, with the addition of the spacer-hub and special adapter it works much like the Tru-Turn adapter.

Other than that you can have one machined as suggested... This is realatively common on Gassers.