I can address one issue right up front. The "new" for '67 A-bodies were not "widened to accommodate the B/RB/Hemi engine families ". Stuffing a BB into one of them tiny things in 1968 was a last min. response to the very fast changing market for musclecars.
I don't think it was even in their wildest imagination when they were working on the redisign years earlier.

"You want to stuff a WHAT into that Dart???"