This kit might fit:
http://cgi.ebay.com/ebaymotors/All-Balls-rear-wheel-axle-bearings-Yamaha-RD400-76-79-_W0QQcmdZViewItemQQhashZitem3f090a8b21QQitemZ270734625569QQptZMotorcyclesQ5fPartsQ5fAccessories
This is were i bought the bearings and seals, its in the UK so i dunno how long you can wait...