A call to SMI will get you the dimensions of the control box and possibly some help locating it if they've seen or done one yet. I've had great help out of them for the AF-1 on our rig. As far as the firewall is concerned, it's just that....a wall to stop fire from getting from the engine compartment to the passenger area, so it should be sealed. Some times you can sneak a wire or two or a small air line through an existing grommet or along side the steering shaft, but in my last two AF-1 installs, I had to find a clear area, drill the firewall and then use a rubber (silicon) grommet. Finding the clear area was a PITA but it was do-able.
Good luck. We've had the same AF-1 on 2 coaches and 3 toads, and I'd never use anything else.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad