The law shouldn't even be an issue in deciding this. Some good aftermarket seatbelts will cost about $25 at a parts store. Your life and personal safey is priceless. It's not that you're a terrible driver, the roads are getting more and more crowded with idiots.
The first thing I did when I got my '64 was buy seatbelts. The threaded mounts were already in the floor and my '67 had factory seatbelts.
To answer your question, '68 was the federal cutoff year, IIRC, but I found it best to check your state's seatbelt laws.
'64 J200 thriftside
'67 J2000 townside custom cab
'68 Jeepster Commando
'69 Super Wagoneer
'79 Cherokee S WT
'61 Aristocrat Lil Loafer