I asked a similar question on a 3 1/2" gauge Betty locomotive tender because I was concerned about heat distortion of the platework.
These were the tips I received – by the way, I haven't tried any of them yet;
1. Try and get some lead solder with a low melt point because the newer lead free variety has a higher melting temperature. The higher the temperature, the greater the distortion risk. You can still get hold of it from some suppliers.
2. Tin the parts first and check that brass angle is truly 90 degrees – sometimes it isn't.
3. One tip included assembling the parts, painting it with solder paste and at least pre-warming the assembly in a domestic oven. Take the wrapped parts out of the oven and use an electric iron to run along the joints. Put it back in the oven between sections and return it to the cooling oven after you have finished so the whole assembly cools down at the same rate – hence preventing distortion.
I am also building Rob Roy and the cabs and tanks of the real don't appear to have lines of snap head rivets on the outside. The smokebox and buffer beams appear to be heavily riveted but the platework appears to be flush.
Another tip. Take care if you rivet the plates with the snap heads on the inside of the platework. My Betty tender was done like that in accordance with the so called words and music and a very neat job has been done of it. Unfortunately, even very careful riveting from the previous builder has left a very noticeable 'dimple' on the outside of the platework at every single rivet position. It can probably be put right with car body filler but it is something else to bear in mind.
Phil H