bench seats 54 4 door crestline
 

what is the most updated bench seats front and rear that I can get without very much modifications

Re: bench seats 54 4 door crestline
 

I can't answer to your car specifically but re my 55 Customline . . . I've measured Crown Vic seats from the 1990s and it seems like their front bench would fit quite well. For the rear I would take the upholstery off and adjust it to the 55 seat frame. Once I'm back to doing my 55 that is the "plan".