If you can't get load/inflation data, go with the chalk mark procedure (chalk across the tread, then move the truck to check whether the pattern is even - chalk left on the outside of the tread means overinflated for the load, and chalk left on the inside means underinflated.) Under-inflation is way worse than over-inflation - you probably recall the Ford Exploder episodes.
Yes, tires will only have max inflation pressure on the sidewall markings.
As I'm recalling, 32-35 psi was the "standard" tire pressure back in the day. That's what I used on my '51 with 650-16 tires. It steers OK and not a terrible ride.
