The ST400 rear mount on the tail is further back, so crossmember needs to be moved back. The 65-6 frame bracket is too far forward to fit it. You'll have to fab an extension, it could be as simple as bolting/welding on a piece of angle iron to the frame, or cutting and extending or moving the original back.
The 65-6 console shifter needs some mods as well. The lever under the floor will hit the ST400 tail. You can make clearance by bending/grinding the original, or use a cable conversion from Shiftworks.
The 67 GS shifter (with ST400) passes thru the floor in a different location, it doesn't use the same floor opening.
Use the torque converter from the 2 speed GS, it's a smaller diameter and has a higher stall speed.
For the rebuild kit, I had a Red Eagle and Kolene kit bookmarked, it was likely recommended by someone somewhere!
