[77newell]

Sadly knowing the weights on the tires is the only way to know the required tire pressure. If I were in your situation I would inflate all of them to the max pressure shown on the tires. I would depart on my trip and stop at the first available truck scale aND get my axle weights. I would use the tire pressure chart for your tires and inflate accordingly. You need to increase the pressures somewhat to account for uneven loading side to side. Other owners of your vintage may have some insight on that figure, I'm clueless.