I concur with MOG615.
I have a 1980 +8 with a 3.9. Driven all over Europe and in 44C ambient temp. Survived with a header tank, Ali rad, bigger fan and over-ride switch. It's not boiled in 200+k miles. Given what you have done so far it needs to be an Ali Rad and override. I used a infra red temp gun and my gauge was running 10C over.
To check the fan is sucking. Turn the fan on and see if a handkerchief is sucked to the rad.
