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Regarding coal conversions, I did not do many of them in my rookie years, but I did remove old oil burners from coal conversions in order to build a new combustion chamber out of fire brick and install new oil burners like the Carlin CRD100 in the 1970s.

The first step is to remove the grates that hold the solid fuel (coal/wood) which was already done when I "modernized" the boilers I worked on. Next thing is to build the combustion chamber floor with insulated fire brick. After that you would build the rear wall and side walls as you might build a wall by staggering the bricks so all the vertical lines were not lined up. At the corner you would also stagger the bricks until you were at the front of the chamber where you would use a piece of 4" vent pipe to leave an opening for the burner's fire tube. Once that was in place you would finish the chamber walls so the chamber was just above the lowest part of the water jacket of the boiler. That part of the boiler was called the mud legs.

With the chamber wall slightly above the mud legs of the boiler you then take vermiculite to backfill the hollow space between the chamber and the boiler base. This is where you find that sealing up the front clean out doors and ash pit openings are important. the vermiculite would insulate the fire chamber from the exterior walls of the boiler base. It would also add a little support for the chamber so it would not fall apart after the fire cured the mortar between the fire bricks. A well built chamber was like an archway where the pressure from the backfill would hold the bricks from collapsing out and wedging the bricks together would keep the chamber from collapsing inward. Some mechanics were better at this than others. Once the fire chamber was completed and the backfill was pressed into place, you would cap off the chamber's backfill insulation with some high temperature retort cement. That would cover the loose insulation at the top of the backfill with a seal between the top of the fire brick and the side wall of the boiler's wet jacket, high enough above the mud leg to protect the mud legs from the intense heat of the burner flame.

Here is a video of how one might build a round combustion chamber by hand https://www.youtube.com/shorts/Ny5q23aXy_U

With today's precast ceramic fiber combustion chamber kits, you can cut the time from 2 days to 2 hours to build a combustion chamber in an old coal conversion.

You first need to build a chamber . Either build it with brick or prefab . I would build it with brick .You need to learn the proper BTU needed to the size chamber . Shape of chamber I would recommend a "Pear" A gas burner Carlin or Riello ... Figure it out before you start