Hi Tony:
I have soldered my Austiin Healey 100-4 rad in a similar place and found the brass sheet metal it is made of to be fairly heavy gauge brass . I used a propane torch to heat it with and found that if my brass was clean (emery cloth or sand paper or file and a little soldering paste) the solder (50/50 lead/tin) flowed easily long before the brass turned red. If you don't like the way it turns out, clean it and do it again. I cut holes in my rusted gas tank and soldered it together after scraping the rust out . It worked well for 8 years.
Dave Potter
