Sorry Jerry for the slow reply I don't often look down here
Normally the clack screws into a a bronze bush silver brazed into the boiler barrel if it's a copper boiler or a steel bush welded in if it's a steel boiler.
On my 3" Burrell Devonshire Engine I opted for a steel boiler a) for the strength given the fact that the boiler is in effect the chassis and b) a copper boiler for 3" scale engine would need to be made from 4mm copper which is way beyond the heating resources I have.
My steel boiler had pads welded on the outside of the boiler for the studs securing the cylinder block and motion bracket. The the holes for the studs were drilled and tapped only into the pad, not the boiler barrel.
I might add that here in Australia we are guided by the 3 AMBSC Miniature Boiler codes ie Copper, Steel and now Duplex. I followed the Copper code for designing the copper boiler for my Britannia 'Boadicea' and the steel code for the Briggs boiler I designed for my Burrell, being steel the boiler had to constructed by a certificated pressure vessel welder.
the photo of my Burrell under construction shows the bush for the clack vale on the side of the boiler along with the pads for the cylinder block and motion bracket..
On the Plastow 3" Burrell owned by a friend, whom I have been helping to make bits for, the clack is fixed directly to the outside of the copper boiler barrel. I haven't seen any drawings, as he bought the engine second hand, but presume there is (at least should be) a doubler on the inside of the barrel as the studs are screwed directly into the copper.
Personally, I was expecting a pad on the outside with the clack face flat, but was surprised to see, when reseating the clack ball for him, that the clack face was curved to match the boiler's curve. Not, IMHO, a very good design.
There's no end to the good you can do in the world - so long as you don't mind who gets the credit