Judging from the pics (which are worth a thousand words) I'd completely re-do the transom from scratch . The idea of s/s rods would just be a band aid IMO
Bear in mind that a 225 Merc is not only much heavier than what the max recommended HP was for that boat back when it was built but it's also a minimum of 100hp more than factory recommended also. Could be more even as the OBs back then were crankshaft rated and the newer ones are supposed to be PSHP rated .
Just because something's old doesn't mean you throw it away !