The original engine was a nailhead, the bellhousing pattern will be different, and therefore the trans won't bolt to the new engine. No doubt there are adaptors made, and of course you could always trade in a newer trans with the older switch-pitch converter parts swapped into the newer case. You'll want to examine the oil pan to assure the sump will clear the crossmember. Beyond that, I have no idea about the complexity of changing to the more modern engine.
