Last time I checked that adapter is still available new from GM, not an inexpensive piece though. 4X Heaven has a better price. Used setups can be a bit scarce but they are around. I do know that the 350/205 splined adapter sleeve is not available new from GM. Lastly here is a short thread relating to this combo.
http://67-72chevytrucks.com/vboard/s...threadid=33702