I've never done a install that didn't have the anchor nuts and reinforcements in place. But as far as the attachments behind the seat, you could add, say, a 3x3" floating reinforcement plate behind and in the tunnel with the nut tacked on.

The sill attachments will be more tricky, as you will need the same kind of reinforcement as the tunnel, but you likely can't get behind the sill. You'd have to cut a slot, say 1x4", make a heavy plate, shape it, slip it in the slot, pull it up and reweld it in place with the nut attached. I'm thinking the floor pan material on the older car will be more than heavy enough to keep the plate in place during an event.

Find a 67 and above GM to harvest the belts and reel retractor out of. Or buy new from the many suppliers. You'll want a retractor at the sill IF it will clear the door, no clearance, then just a loose belt with a 1965 style retractor, and a sliding buckle on the tunnel side. You'll be glad you have any kind of retractor to keep the loose belt out of the closing door. Use grade 8, 1/2-20 bolts and nuts, and SAE washers if you don't have the factory fasteners. Most belts will have at least a 1/2" hole in the mounting plate.

The only concern is how the buckle side will pass through the seat. On 65 and up cars, a slot was provided in the bench for this. Maybe it will just pass up under the seatback? Cheers and good luck.

I think seat belts were an option before 58 so the floor brace would have the welded nut for the seat belts. They would have allowed for only 2 belts in front. If you have a 58 Chev assembly manual, it should show dealt belt option in the RPO section at the rear. This would work for factory lap belts.