I think 95/5 solder would be ideal for your application. That solder is not going to melt at 365 degrees F like normal electrical solder. 95/5 melts at just over 460 degrees F. However, to get good wetting to copper in the radiator, the joint should be heated to around 525 degrees F. If you don't get the repair area that hot, you'll have a cold solder joint. Solder has to not only melt but also dissolve the base material you are soldering to. Using plenty of flux to prevent oxidation and aid in the wetting process is mandatory. After your repair is complete, clean the area well with isopropyl alcohol and then water to deactivate the flux. If you don't clean off the flux, you'll get corrosion and failure at a later time.
BTW: In aircraft, high temperature electrical solder joints are often a solder called Sb5. It's 95% tin and 5% antimony. The small bit of antimony keeps the tin from crumbling due to wide range temperature cycles like extreme cold at high altitude vs ground altitude. When tin crumbles this way it's call tin "pesting" and is NOT something you want in electrical wiring harnesses on an airplane.
