The "Ply Rating" is that, a rating not a construction detail. Back in the days of fabric bias-ply tires, "Eight Ply" was a heavier duty tire for 3/4 and one-to trucks. Possibly to get away from questions like yours, they've adopted Load Range to replace Ply Rating. The Load Range can be achieved various ways.
65PSI confirms you have Load Range "D" tires now, so it's a 3500 (or 350) "One-Ton" (another term hard to relate to actual carrying capability) chassis. The 450/4500 is "Ton and a Half" and requires Load Range "E."
You'll have to go out of your way to find tires in your size that are not "E" since there's no penalty for using them in place of "D" meaning why stock them. Just be careful they are in fact Light Truck (LT) series since that size is also available in Passenger (P) series. So you're shopping for LT225/75R16E.
Because a "D" tire will handle the capacity of your chassis, replacing with "E" will give you a tougher tire. That's why I don't think you need to go with commercial-duty tires like many of us with 29-32-foot Class C's have done.
