I'd check the local transmission shops. When I needed a 518 case for mockup on my hot rod I went to the shop down the road and picked up a bare case, tail housing and pan for $50 and they even ran it thru their parts washer for me.

1957 Plymouth (Hemi, Dual Quads, A833 4 Speed 9 1/4 w 4.10) Sold
1937 Dodge Pickup (Hemi, 6X2 intake, 46RH, Dana 60 w 4.56) Sold
1968 Plymouth Valiant 2dr sedan (354 HEMI, 46RH w/4.30 gears) under construction