This is what was published by the Tire and Rim Association in 1954. It is for tube required bias tires.
https://photos.app.goo.gl/9s6PwtcUaPD1mCwCAIt looks like the minimum for a 7.50x20 8 ply is 40 psi
The minimum for 7.50x20 is 55 psi
You must remember that the old bias tires were not made for the sidewalls to flex like modern radials. Even with no load, bias tires flexing their sidewalls generates heat. I was already riding in two tons in 1954 and Got my commercial license to drive them, as well as any truck in 1959. If you have 10 ply tires, you are asking for trouble with anything less than 55psi when they are cold.
