I can't find the links right now, but if you search google for DCC installs on this... There should be 2-3 that show how to take it apart.
That bad news is, the smoke box is one of the very last pieces you can removed from the body. My kind of luck.
If I remember correctly, there is no easy way to do it and requires removing your piston and valve assemblies to provide clearance so the smoke box can slide forward. That, of course, is after you completely remove all other pieces of the shell.
I have never worked on one with detail parts installed, so you'll need to address that before starting.
Things I do remember:
-Cab slides straight up, after being pulled slightly out on the sides.
-Main boiler should be gently pulled up slightly at the rear before the front, and be VERY CAREFUL not to brake the tiny tabs off the bottom of the front of the main boiler.
-The ladders on the pilot have tiny pins on the bottom and must be pushed slightly upward before being 'popped' forward to allow the pilot to slide off.
-If you remove the piston and valve assembly, I hope you're a religious person. You'll be doing a lot of praying and repenting to make up for the words required to get them back together. (and sometimes apart)
I didn't type this to discourage you. I just wanted to point out a few things to be aware of.
Ok, so I found a link which proves me wrong. Here you see him saying you can remove the smoke box at any time.
Wiring For DCC - N Kato Mikado
