Greg,
I can help you some, but I am a less knowledgeable about the big cars. The 67 and 68 Chassis Service Manuals show that the dimensions of the Electra and Wildcat frame are the same as the Le Sabre in the engine cradle area. The Le Sabre frame was shorter relative to the Wildcat and Eelectra frame after the engine cradle. The frames did not change from 67 to 68 in the big cars. I think the same big car frames were used until 1970, but I am not 100% sure. If the same frames were used until 1970, you would need the frame pads and motor mounts from a 67-69 Wildcat or Electra to fit a 400, 430 or 455 into the Le Sabre. The Le Sabre could have a 300 or 400 transmission, and I would swap in the 400 if you did not already have one. Maybe someone else could tell us for sure if the frames were the same in the big cars through 1970.
Steve