I would hammer all over the frame once I had it out from under the car. Rust starts on the inside and chances are there is more you don't realize. easier to find and repair weak spots now than later. Went through this with a 67 el camino I did for my father. Had to swap frames and the doner car we found looked to have a perfect frame while looking at it from below..once it was out from under the truck and it had been sand blasted we found some areas that were thin and needed plated.