What is the width of your 15" rims? I assume you have manual steering. I would look for a larger outside diameter closer to the original size. It allows you to drive faster at lower rpm and it's easier to steer. Look at tires 700R15 or 820R15 from Diamondback tires.
www.dbtires.com. They're sized like bias ply but are radials in the correct original size for the truck. They'll be about 2" larger in diameter than the ones you have now.
Modern radials are generally inflated to about 35psi. I keep 35 psi in all of my vehicles except the Packard. It gets 39psi.
A good way to check if your tires are over-inflated or under-inflated is to drive through dry dust or dry dirt on a concrete drive and look at the treads.
The dust stuck to the tread should extend evenly across the tire. If the edges don't have dirt/dust stuck to them, the tires are over-inflated. If the center of the tire isn't evenly coated with dust, the tires are under-inflated.
