Overload springs are probably available that don't engage until the springs compress somewhat. I have overloads on my F250 that work that way. So they don't affect the ride until the suspension compresses enough so the ends of the overloads hit the brackets attached to the frame, totally separate from the main springs. The same type were standard equipment on some AD series big bolt trucks
They would be quite a bit more difficult to install than air bags. If you're looking for simple install, air bags would be your best choice IMO. The pressure can be adjusted for the load, so if you're unloaded with them aired up to minimum pressure, they wouldn't affect the ride significantly. But like you say, a specific air bag kit for an old truck would be hard to find, and would have to be adapted.
