Josey, How about this...shorter fins tucked in with a semi rounded back area in back...the ends of the fins would be even with the back but that area would round in to give the fins a inside look of being alittle longer - think '57 nomad. Another fin idea is the early '60s caddies..short height/length fins.
On a far left field approach...very far...maybe incorporate canted quad headlights in the front opening....ok, maybe too crazy now...been gluing too much this morning I guess.
fins would be short like factory..leaving the back end area alone,,that could be used as a trail gate/backend.move backwindow and roof ahead and make a new back wall...i was thinking on the headlights too..cant believe the ones GM put on this train..bet the lighting wasnt that great..too much glue..with me its paint fumes..lol..lol.