I put some retractables in my ute, but i had a bit of a problem with the units jamming when the seats were right back, as the seat would rub the belt, and they wouldn't retract. In a sedan, you could probably move them back a bit more...
Depends what sort of seats you have aswell. I don't know what mine are, but they aren't stock.
Also, I haven't tried 120y seatbelts. mine were from a galant i had laying around out the back.