I just completed a hub conversion on my 312-8 with 520 front end.  Being unable to find 520 front wheels, I decided to go with trailer hubs.  I could not find a good write up on the swap, so I am posting this for anyone who wants to do this swap.  All parts (minus tires) came to about $100, so the conversion is around the same price as a set of clean 520 rims. Parts List -Trailer Hub Kit, I bought this off Ebay From Elgin Trailer Parts, This includes 2X 1000lb 4 bolt trailer hubs, bearings, 1"