For LMS use the LNER ones. The SR varient has an additonal flange underneath which I think is unique to that region.
You could also consider the ones from Dart Castings: https://www.dartcastings.co.uk/mjt/2940.php
They are 8'0" Fox Patent bogies. Have a look at this thread: https://www.lner.info/forums/viewtopic.php?t=10472
You are in luck. Removing the duckets will expose the panelling Rapido thoughtfully included underneath.
More info here: https://www.lner.info/forums/viewtopic.php?t=9331
The one time I tried Chinchilla dust it was made of ground clay. When glued the clay dissolved and expanded, then on drying shrank and cracked so I have not touched it since.
You are probably way beyond this point but comparing the 3D model with the Deltic image I would say the side profile is too rounded on the model where it should be more hockey stick. Have doctored your image to exaggerate:
I suspect the gangways might be undersized. You might like to check out
If you go back through the topic you will also find some scale drawings that might help.
Good luck.