I only know a little about them.... I rebuilt one in 1984 or so! Yes, they have TH400 internals. Might want to upgrade the intermediate sprag and I believe sometime in '66 they added a snap ring to keep the center support from moving around. I think the earlier case can be machined for it. Later valve bodies can be swapped in for better control.
'64 ST400 used a different case, valve body, and didn't have the switch-pitch converter. 65-66 units are preferred.