Overheating issue with the old slant 6

^^ The above are a very good set of questions and thingks to check.

Most every car is going to show some temp rise at idle after running for a while. What year/model of 225? This can be important; the shaking at shutoff may be 'dieseling' which was not uncommon for mid-70's model cars when the emissions stuff was first put oin cars; that typically gets worse with a hot engine.

BTW, installing the fan backwards won't reverse the airflow (think about the blade angle); it will still blow towards the engine. But it may place the fan in the wrong spot within the fan shroud opening and not pull air properly at idle.