According to past judging practices, seat belts are allowed as a safety item (some states may still even have mandates for all cars on their books). They could be tucked in under seat cushions if you want a more original appearance when showing.
Interestingly, at one point K-F planned to offer Seat Belts (SB on the 1952-55 Firewall Body Tag, as far as can be determined at present) as an option but the story goes that Nash Motors had issues with prospective customers when they offered them as a dealer installed option during 1949-51 model years. In the midwest area for many years, the belts were the most commonly seen Nash factory-branded part at general swap meets; at this year's grand Nashional there were none to be seen.
