I may be wrong, but I think they are the same, except the models with the power rear seat. I think the extra length was in the rear footwell area. Please correct me if I'm wrong, but I thouth the SEL was about 4-6" longer, mainly in the rear footwell area, with a longer rear door.
My question would have to do with the power/heat available on the later models, and how hard those features would be to integrate, relays, wiring, switches etc. (front seat, doubt reclining rear seat would ever work)
1978 240D Auomatic
Custom Charcoal Gray, Navy MB tex, kenwood/Elemental/Boston/Aura sound,
Shaved antenna and emblems.
1990 Lincoln Town Car, Sleeper/Strip car in progress, 17.75@77mph