In South Africa 520 rear sprockets are not available for the 400,so I buy renthal blanks and machine the centre to fit the 400 hub.
Also need to space out slightly as it's thinner than the oe sprocket. the front sprocket lines up as per oe,as it has a boss to keep it in the same line.
You can order front and rear sprockets from sprocket specialists in the USA, they will make up a rear for you, the front is a stock item I think from the SL350 twin. I heard that Rick Denoon was going to make up some complete 520 kits for the 400 Four, I think he is a member here.
Where are you Rick?
Kevin.