Yu should be fine loading the already primed cases with mid-range .38 Special loads. Just think about making sure they would not squib if the primer was really the weakest of regular primers, not get grossly over-pressure if it was one of the hottest magnum primers. That should leave you lots of room for choosing a load. And, if you are going to shoot them is a gun chambered for the .357 Magnum, then of course you would have LOTS of room for error with .38 Special loads on the hot side.
One issue that you might encounter if the primers are REALLY old is that some of the cups might crack when fired. I have had a few 30-year-old primers do that. It leaves a little ring of dark residue around the primer/case junction, like an over-pressure load might do. But, the primer pocket is not stretched, and there is no apparent flame-cutting on the recoil shield, so I don't think they do any significant harm. I have carefully decapped the few primers that showed the dark ring and examined them with a 10x loop, which clearly revealed the crack along the side of the primer cup that leaked the gases. You could check for cracks the same way if light loads leave some dark rings.
SL1