My first attempt at buying a chuck didn't work out too well. I forgot to make sure it was a plain back and it ended up having integral threads. So I had to return it. My second attempt worked out well.
Besides some wear on the ring and a few chipped gear teeth on the pinions it is in good shape. Luckily the chipped teeth don't affect the function as far as I can tell.
Since the 1-1/2" x 8 backing plate is smaller than the 1-3/4" x 8 that the Clausing has I believe I might be able to re-bore, and re-thread the plate. That way I don't have to buy another backing plate.
I did buy a brand new backing plate with the correct nose thread from AMTOOLS but I think I'm going to use that on a new 4 jaw chuck when I get one.