I can see one from my yard too, came from an ex Snap-On truck..
There out there, keep looking.
Try a wanted ad on http://www.4btswaps.com/ These folks often scrap step vans for the engine, the axles are generally unwanted and reasonably priced. Remember now, not all step vans have the desirable axle, so you'll still have to look for the right one.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-