There are three versions of the step casting, but they all have the same threads for the dolphin striker post.

Make sure the threads on the post are clean (wire brush) since there's undoubtably corrosion on a 36 year old post. Use a lubricant (WD40) when you thread the casting onto the post and back it off 1/4 turn for every 1/2 turn (like you're using a tap / die).