The easiest way is file flats on the front spindle, only needed where the spindle goes in the fork end.
From the mid 1950's most forged front fork ends were 9mm, and QR hubs were mostly 9mm. BH made front hubs with 3/8 inch for their QR (we used to file off the tops of the threads where the spindle sticks out from the lock nuts), and Raleigh made millions of bikes with flatted 3/8 front spindles. Also British tandem hubs were normally 3/8.