Probably shouldn't admit to using leaded solder given how the last thread went.extrusol
But it does protect the joint from vibration and stress, but there you go done a few in my time top and bottom tanks off as well to clean out cores out the cores, but what do I know
yep simple enough hardest thing to solder is stainless steel it must be perfectly clean and use an acid fluxSoldering a brass radiator is no different to soldering a plumbing fitting i've modified 2 honda cx500 rads and just used what ever flux i had and a stick of what ever solder i had at the time and for heat it was a little camping gas torch. Dead simple. Very similar your your photo.
Flux emProbably shouldn't admit to using leaded solder given how the last thread went.
Sometimes it can look rough and work.As above hardest part to soldering Is cleanliness.
Only pipes I've had leak has been modifying existing, you can tell when it hasn't worked as it's messy and crappy looking