You are blessed with good neighbours where you live.
I believe that Jack Stands are rated as pairs.
But, trying to rate your own, you would have to destroy one of the pair as you would have to load them to breaking point. Then, halve the weight that caused the first pair to collapse, and you would then have the load limit of the remaining pair.
If the rust is not structural, then you should be relatively safe to use them under your car, as 4 would safely hold up a 3 ton vehicle.
A good engineer could cast their eyes over them and I am pretty sure they would pass with flying colours.