This is a 4speed car, not a TH400 car. To my knowledge the TH400 crossmember is the same regardless of the engine size.
All Yenko Deuces came with a BB crossmember, and you are right it doesn't make sense. BTW, it's not a Pontiac member either, which looks exactly like the BB one but doesn't have the 1/2" offset. We do not know why they came this way, but they did, and they do not have the BB engine mounts - which makes the trans mount fun to install
One theory that we have is that the BB member was included in COPO 9737. #9737 had been a BB specific COPO since '68, and when it was modified for the Deuces in '70, the BB member was not changed to the SB one. It's a long shot, but right now it's our only shot.