Originally Posted by CO_Hoya

Just to be clear, do you mean that you have a freehub with HG cassette, or a hub with a 6sp threaded on HG freewheel?

Also, at least one version of the Superbe Pro RD is reported to handle 28t. If 42/28 (39" development) is acceptable, a simple/cheap change in the back is probably the way to go.