I have seat belts in all my cars, and use them. But, I don't think it's required by law to install seat belts in cars not originally equipped by them. I would install a lap belt in your old cars at a minimum. But a three point belt only if it's easily doable.
Unfortunately here in Indiana no matter if the car came with belts or not it is required. I have lap belts in the cars now. Looking to upgrade that’s all
Yeah, good luck with that when children are involved. It's OK to lop their pecker and put 'em in a dress, but if you don't want to get stopped every 15 minutes, better get lap belts installed.
A HAMB-relevant vehicle would not be required to have a safety belt per FMVSS, which the Indiana Code references in its safety belt statute, nor would a such a vehicle have had safety belts installed as standard equipment by the manufacturer. http://iga.in.gov/legislative/laws/2018/ic/titles/009/#9-19-10-2 You might get stopped, but law enforcement will have no legal grounds to cite you. Stating that belts aren't required isn't advocating that belts not be installed an used.
I asked a couple of local cops about having to use a child seat in my 47 even though it never had belts. They told me I had to have one if the Grandson rode in the car, even if nobody else was belted in. So, I put in a belt to fasten down his car seat. The car was exempt from having to have belts, but the child seat law doesn’t recognize the fact that old cars never had belts. Apparently if a kid rides in an old car, they have to have a car seat. It’s a kind gray area of the law nobody talks about.