I got the job done yesterday - went pretty much as you said, Ayjay - the flamethrower I'd bought was just that - but even then, with me having to heat up the really thick left-over tab of the previous job and the rad soaking up the heat, it took a while, but once heated up, it stayed hot enough for easy adjustments.
Funny thing was, it got so hot that the glue that they had mounted the rad protector on with burst into flames, and that fell off, too!
I just burned off the remains and used chemical metal to stick it back on. Used that to reinforce the welds, too, and I'm very happy with the results. It hasn't been fully up to pressure yet, but some short running tests were successful, without leaks. Also used the opportunity to do several other under the covers jobs and give it a quick spit n polish. Hasn't looked this clean in ages...
Thanks all for the input.