I have made the smaller center flue with cross-tubes boiler described in Tubal cain’s Building Simple Steam Engine Book 2, though I haven’t completed the fittings yet.
Described as suitable for a twin cylinder engine of 5/8″ bore, I’ve assumed it is plenty big enough for my stationary engines, none of which is above 3/4″ bore, single cylinder or expected to do ‘real work’. It is 3″ diameter by 5 1/2″ high (boiler only), so a lot smaller than yours. He suggests adapting a Camping Gaz burner or a Cheddar Models Gas Diffuser burner.
I found it easier to use the central part of a three-legged epigas burner (the low type is much easier for camping use too!) Without the legs and with the holes drilled out the burner screws direct to the baseboard, witha decent length of metal pipe outside the base before the rubber pipe joins it. The original combined tap and adaptor can still be used.
I would compare the burner to a typical middle sized domestic cooker burner, and I’m sure it would fire a boiler of your size, though perhaps not absolutely flat out.
Neil