Thanks Tony,
I suppose it does depend on how the item, chimney or any other part, is located. You obviously know these kits far better than I do. I would make a hole from inside the smoke box to allow a good route for the solder to flow.I have always worked on the principle of doing as much work on each item as possible before joining the individual items into a larger assembly. Model railway stock or buildings to Ikea units I find it a good maxim. it might well require more time spent at the thinking stage to get the best out of I.
Bernard