I don't know the actual weight of either motor, but seems like when we pulled the original straight 6, it was a heavy SOB. However, interestingly enough, whoever had this thing welded in leaf springs in the front end.

Considering my '68 has coil springs, I'm pretty certain the leaf springs on the '67 are NOT original, although I don't know jack about the '67's.
So, it may take some investigating to figure out what the heck to do to fix the sag. May just be a case of needing to add an extra leaf, or replace the existing with stronger ones. I still don't understand why they would switch to leaf springs though... if weight was an issue, you can get however heavy duty of a coil spring that you need. Really bizarre.....
Thanks for the reply and the help!