All Sud-Est Armagnac were broken up in the fifties. Setting up production would be a hazard and I doubt the range / capasity is sufficient.
Folding up seats on the 767 seem less praticle. Its 3 wide seats in the 767 instead of two narrow ones for a start, apart from that, not to many 767s are used on short haul..
I agree on the frontal area / friction & weight on the 767 vs 757. Thats why I tried to introduce narrow body efficiency with wide-body comfort and twin aisle mobility.
Apart from that I hear the 757-300 offers great CASM but is on the edge regarding stiffness, comfort and (de)boarding. 20 inches additional cross section could do the trick offering new cabin options and operational flexibility without introducing widebodies cost levels.
For longhaul a two class configuration seems most feasible. 4 class is possible but likely few routes would be able to support the higher CASM.