J...
I am enjoying your journey more and more.... reminds me very much of mine over the years. I imagine seeing the first few drops out of your still, you're going to about shit yourself (God knows I did!)
I too am no expert in soldering..... almost all my soldering has been for this fine hobby, but I have learned a few things along the way. Like BDF mentioned, the small parts of stainless to copper are very similar to copper to copper soldering. Indirect heating is the key. You should not directly heat the rod but rather allow it to fill the joint with indirect heat. When I do stainless I do use Harris Stay Brite... makes short work of any stainless steel job. Don't breathe the fumes...
While you were soldering I get the feeling that you were just trying to "cover the gap" (AKA just get solder on the gap). Remember the flux you use helps to actually draw the solder UP INTO THE JOINT. This allows for a good seal. In the beginning, you can be liberal with your flux(lead-free, water soluble flux I assume....) which will make the soldering a little easier. Apply it thick...
When I solder I usually tip my parts in the vise at a slight angle(35-45 degrees). This allows for the capillarity of the flux to pull the solder down into the joint. To have the ferrule on the bottom and expect the flux to creep UP into the joint against gravity is more work than you need. If you are worried the ferrule will move before you finish then just heat a small spot and solder it. This will hold the pieces in place for the rest of the solder.
Be careful... just because there is no water leaking doesn't mean the ALCOHOL vapor under a small amount of pressure won't find its way out. Be sure to pay close attention during your cleaning runs for pinhole leaks.... they're a true pain in the ass if I ever saw one!!!
Also.... there are many great videos on YOUTUBE (you know.....youtube.....?) that can help with soldering/brazing. Many are done by companies who create the soldering products... so there are somewhat content experts. I watched a few before I started(and made a big mess). Of course, there is something to be said about making mistakes and learning....
I hope these little tips will be helpful and I hope the WAY more advanced folks here will drop in to give good info and resources for ya. I promise the more you do the better you will get. That will mean fewer times you will find yourself fixing the pinhole leaks.