I don't believe that the frames of that era were made of high strength steel. Typically just low-carbon steel. But welding on even low carbon steel will affect the strength in what's called the heat affected zone (HAZ) of a weld, which is typically where a crack would form. Generally speaking, welding on the flange of a frame is not recommended, as cracks can start at the edge near a weld and propagate through the frame section. The same is true of drilling holes, which should only be done on the web (the vertical part) of a frame.
That being said, if you don't have any holes rusted through the frame, or cracks in it, it's probably usable. Welding at low stress points (like at the tail) is probably safe as the loads there are minimal. But ANY welding at high stress points like near spring mounts should be avoided.
