1981 Camaro/firebird rear axle is the correct width and comes with disc brakes. Modifications to the spring perches will be required. To my knowledge there is no direct bolt in option. If the disc brake requirement is flexible then any 74-81 GM F-body (camaro style) will work well. This is what I have used on my 54. The flange to flange width you are looking for is 60". I have heard that lincoln rear axles are also used in this swap but I am not sure which years are applicable. Check out this link for more information.
http://50chevy.freeservers.com/