Nice drawing, really nice drawing
I'm not only wondering about axle weights, but also the ability of this machine to go around curves with that trailing truck. I think I'd arrange it to be a 2-8-4T with a Class 5 boiler and standard tanks and bunker.
The Brighton 2-6-4T had its origins in an LMS design of ?1941? which wasn't implemented. The idea being to give universal use of a Class 4 tank engine as the then Fowler and Stanier designs were out of gauge for some lines, so replacing obsolete 4-4-0 and 0-6-0 designs being used for secondary services. The need for such an engine became urgent on the post war SR, so Brighton took standard BR components, added a few more, and modified the cylinders to clear the restricted loading gauge on some SR lines, and put them back together again.
I wonder how your imaginative (and I mean that nicely) locomotive would cope