I think I see your frustration. The lack of a frame section between the motor brick and the front cylinders does pose a bit of a problem. You might take another look at a B’mann 4-4-0, and see what you can do to fill in that space.
One of the problems, when starting with a Hartland/Kalamazoo 4-4-0 as the base of your project, is that the model starts out rather short in length, and is based on hiding that motor brick. It is also based on something around or close to 1:24 scale, while the cylinders you are using are somewhere around 1:22.5, and may seem to appear oversized, with their steam chests.
But…don’t give up; you may pull it off yet…take a break for a day or two, and explore your options. You are gaining skills and experience as you work, and can do it, if you keep trying.
Fred Mills