Hi Brian,
I would not bother with tinning the surfaces.
Just rivet it up tight metal to metal then do what most of us do for loco side tanks and tenders…
Get some Bakers Fluid flux (easiest these days to get off ebay),
and some old fashioned plumbers stick solder (I have a large box of it if you want any).
Heat up gently, spoon in a few generous large spoonfools of Bakers Fuild and swish around. I use an old brush though you can make a copper wire brush quite easily. Then add the stick solder. Add more Bakers Fluid as required. The soft solder will quickly wizz though all the joints and seal perfectly in less time it took me to type this.
Wash and scrub the joints thoroughly afterwards with a toothbrush and hot soapy water. You may need to do this a few times a few weeks apart.
Cheers,
Julian