Good advice above...
I think the biggest problem that "aged" tires will give you is speed. Flexing makes heat. Fast flexing makes lots of heat, slow flexing makes very little.
To overload a tire in a controllable, off road, lower gears type situation, no worries, These tires are tough. You aren't going to weaken or harm them by overloading in that context.
On the road however..... when they get old sometimes they'll pop just to make sure that you're on your toes. You really want to watch the ratings, and you really want to feel the tires, frequently at first. Old bias tube tires do run warmer than new stuff, but when they reach their limit they start heating up a lot. I figure the front axle when empty is not overloaded, but loaded pretty well, so I use those as a reference for my limit. I havn't reached it, or even close to it, but that's where I set my limit.
FWIW, I just looked at several brands of 9.00-20's that I own, all state 4000 pounds single, 3510 dual.
