Interesting design. Like the others, I particularly dig the assault gun/tank hunter option. My first thought was KV-2 as well...
I think a nice idea (as an option?) would be an ammo crane like the Sturmtiger, etc. So that if you use the heavy assault gun barrel (the short, fat one), you can add a crane to the decking somewhere.

I don't play 40K and honestly couldn't see myself buying one of these for any real reason, but it's definitely got some charm to it.