Hi
Build looking good . I have built a few of theses type of kits over the years and would suggest the following re the body springing .
A) Add two brass bulkheads where the cab bulkheads would be ,helps with stiffing and the body profile . Also could add stringers further along to maintain the profile and helps with the following .
B) I usually add a floor out of thin ply to fit between the lower body also helps with the stiffing , the bogies can then be attached plus any underfloor pieces to the floor pan . Floor would sit between the bulkheads attached with bolts screws etc
leaving cab area free . Helps with running in and wiring ie can just detach the body and test run to solve any faults etc
C) Re cab roof castings Dave Parkins original producer of these kits has mentioned some were to solder the Whitemetal casting from the outside as the best method of attaching to the cab sides . As I fell into the trap of soldering from the inside and often made a pigs ear of it I would certainly take his advice .
All the best
Andrew