Jeffrey, P30 frame rails should all be the same. Difference would be in the application. Mine is a COE, same as the 1980 truck was a step-van/COE. So the steering box angles up. A conventional long hood truck may have a different steering box, or at least it may be angled different? I believe other big truck makers use the same frame rails, but may have them wider apart due-to there axle & cab mount width. I don't know that much about them, I was surprised to see my 1939 was the same as 1980. I believe P30 frame rails are still used, the axles & cross members may differ.
I once saw a 1946 2 ton conventional restoration that the owner unbolted the original straight axle & mounted an independent front axle in place. I'm sure he needed to drill holes, he may had boxed the rails, I don't remember the details but it worked.
