I agree BIG,they
are better than nothing, they will stop you being thrown out in the event of a big one.
At least it gives the paramedics a chance when you're kept 'all together'
inside the cab!

My buddy had a 70+mph spin in his '52 with three in the cab.(NO seatbelts)
Thankfully the doors stayed shut,no-one got hurt and the truck wasn't too badly damaged either.
Seat belts were fitted shortly thereafter.
Here's a little 'factoid' and sobering thought.
Even a collision at as little as 15mph, no-one,no matter how big or strong they are, can hold themselves back off the steering wheel in the event of a bump.