Thought I was going well with manufacture and dry assembly of boiler until I came to try the water and blower pipes that pass over the firebox and they would not. Found that I had allowed a bit more length on the endplates (4 13/16 rather than 4 5/8" thinking I would trim it off later, forgot about it and measured centre distance for the firehole ring etc up from the bottom edge resulting in it sitting too high.
One of these 'I could have kicked myself' moments, stopping to think about it before measuring and drilling may have avoided the problem.
Do not attempt to cut out the firehole ring in the backhead until the inner firebox has been completed and crown girder stays silver soldered to the outer wrapper.
Glad to hear you have a solution via more copper plate.