This is my second effort at a boiler the first a multitube vertical boiler was straight forward enough.
This might require a little more thought.
Mark,
Here are a few pics of a Scotch Marine boiler which may help you with your thoughts.
This one is 4.75" dia with a tin can type fire box at the front to take a water heater and one at the back to take a steam dryer.
They are quite fast steaming boilers and once up to pressure with a feed pump will happily stay at 60 p.s.i. all day, my burner is converted plumbers blow lamp.
Hope this is of help George.