Mick,
I'm currently building a Rob Roy boiler (mostly) according to the drawings. I bought a kit from Blackgates with flanged plates for slightly over £200. I took my latest efforts to my inspector last night.

Currently, the firebox and foundation ring pieces are held together with 10 BA nuts and screws – these will be replaced by 1/16" rivets before silver soldering. I'm proposing to to use 1/8" dome head rivets for the side stays, head on the inside and the outer just poking through and silver soldered. The inspector is happy with that, I think it is generally preferred to nuts and caulking these days. As you can see the, I extended the sides after opening up the tube. In the interests of symmetry I did both sides the same. This was a mistake as the butt strap interferes with the foundation ring and I had to mill a tapered recess in the foundation bar to make it fit.
It would have been much easier to leave one side long and to have a butt strap on the other side much higher up and out of the way.
Making the crown stays to fit is also quite awkward to get a good fit.

Long 1/8" rivets would be much easier and my boiler inspector suggested using these – but as I've made the sheet crown stays now I'll use these and they will be soldered to the firebox crown before the firebox and tube assembly is finally inserted into the boiler tube.
Not he world's neatest boiler but, so far, it looks like it might be OK.
HTH,
Rod