[Mr70]

The CJ prefix TH400 is indeed a 1969 396 series 10/20 Truck application.
Not all the trucks used the long tail shafts.Certain series of trucks didn't,and I believe this is one of them.
Can't say as I've ever seen a TH400 with a 3 bolt convertor originally,only the TH350's.
Do the 3 each have a triangular bolt strap welded behind them?
What's the outside Diameter?..it should be less then 13-1/8".
By chance does the convertor say,"Rebuilt By GM" ink stamped on the front of it?