Seat belt laws aren't retroactive. If the vehicle did dot come with shoulder belts, you aren't required to add them. That '68 stuff is about cars... trucks and vans (and motorhomes) typically had no shoulder belts until much later.
You can even legally drive with no seat belts at all if you are in a car old enough to not come with any, generally pre-'62.
Now, if you do add belts to a car that didn't originally have them, you are then required to use them.
So, if your rig has all the belts it originally came with, even if it's just lap belts, you're legal.
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