Bolt Size For Early A-Body Seat Tracks?

I'm not sure about earlier tracks but... 67 and later did have 5/16 bolts. A standard 5/16 bolt will have a 1/2 hex head. A thin wall socket wouldn't fit in the track with that size head. So...
Some examples had special made hex head bolts with smaller heads. Others had a special round head bolt with a phillips like posi drive. I think a allen socket head bolt might work there if you need to find "off the shelf" substitute, Unless that socket head is too tall to allow track movement. I haven't tried it. Good luck