Chevy shop manual says 35 psi for a 15 in. tire.
However, this is for bias ply tubeless tires. I run 35 in the radials on my '66. You don't want to fill the tire to "max" pressure. The truck will ride like a brick!! Not to mention tire wear issues. You'll be fine with 35 psi all around. For heavy duty operation, you could increase pressure by 5 psi. Just don't forget to adjust the pressures back to 35 psi when unlaiden.
Jim Sgrig
