Frustrating. I don't have a good answer. Shortening the frames to draw the beam to the footplate is not much of a solution. Scabbing on more brass will not be easy, but may be your only choice.
Regarding the frames: Are they to be sprung? I think I see Slater's sprung axle guides. Are the coupling rods rigid? If so, they will compromise the springing action. Premier make a nice set of coupling rods for the Cauliflower. I have a set for my kit, which is waiting at least for you to finish this narrative. They are much thicker than the rods you have there, closer to prototype, and they are nicely fluted. The tricky bit is that they are prototypically jointed. In 4mm the cunvention is to joint the rods on the crank pins. The prototype does not do this often (the LNWR did on some 8 coupled) but rather had a joint next to the center pin. The Premier rods have a very nicely machined joint, and a steel rivet. I have used them on three locos. And soldered up the joints solid twice. needing unsoldering and cleaning up.
For the coal tank I got it right: I dry assembled the three parts, and applied a small drop of oil to the pin from the front, and a tiny drop of flux from the back. This prevents the solder creeping forward to the front half of the rod. When the rod drops of its own weight when held horizontally, you have it right.
I assume you used jig axles to set up the axle guides. You will need to check the spacing if you use another set of rods such as a Premier set.
Chassis assembly is a delicate bit of work. I learned how in 4mm, and have had good luck with my four kits since switching to 7mm.
How are you picking up power? I can commend plungers, but they are more easily drilled for while the chassis is in the flat.
Kevin
