stainless steel soldering

Did some cleaning of the stainless steel hemispheres, they look much better now, I really had some luck finding them.
Exept for one hole each, they have the right edges for standard viton rings.
The opening without the right edge is also damaged, so I will solder a copper ring on it, to solder stainless steel (I think it's 304, but it might be 316, can anyone tell my how to see the difference?) I used lead-free solder (SnAg, 3.5% Ag), and S39-rvs/inox.
Soldered it with a small torch, the soldering was ok, but a bit slow, My torch didn't have enough power.
Next step will be pre-heating the whole thing with a fire and then do the soldering.
Here is a picture, with the soldering halfway.