I am looking at doing the same thing with a 4320 with 18.4 38's.  Best I have found so far is http://www.woldinc.com/index.html  They have a single bevel that will fit the 38" cast center and use a 320/54 tire.  It will sit about 2" taller than the 18.4 38.  They are 1200 per rim.  Still looking for a used setup but having no luck so far.

Gunner