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Here is an design manual for Dexter axles, if you are comforable going with that brand. http://i.b5z.net/i/u/1080235/f/Appl_Manual_4-04.pdf
In addition to Jim's suggestion of 35 degree down angle, you can get the Dexter axles with "high profile" brackets. They give you another 1" of clearance.
Much more than that can affect your center of gravity and make handling less stable in high speed corners. If thats a problem.
It sounds like a fun project.