Unless it has changed since I bought one back about 1990, the SA A-1 will not have the Series 80 type firing pin safety. Also, it sounds like he is replacing, as in putting back together, not as in putting a new part in place. If I have that right, it should go back together.
One question I have is whether you dry-fired the hammer against the slide with the firing pin and firing pin stop removed? Doing so can peen the metal around the mouth of the firing pin tunnel enough to cause problems with either insertion or popping out. If you did that, you will need to use a drill bit or a reamer or a tapered reamer, or a burr remover to clean up the mouth of the tunnel.
Hold the slide so you can confirm that you got the extractor in where it causes no interference with slipping the firing pin stop into place. If in doubt, just pop the extractor out and see if that lets you seat the firing pin properly. If so, you likely had the extractor turned a little counterclockwise.