Well call me strange for suggesting it, but why not use 2 claw-arms with a pair of pegs on the sides of it to mount the cannon and to give the back-pack an secondary original twist? I dunno, but to get an idea of how it would look you could toy around a bit with the claw arms of say THS02 Prime, regular prime or Overlord/etc. depending on your collection as those sorts of claw arms are not hard to duplicate/mod from a different toy.
Also, please consider using "boltgun metal" instead of chrome in the color scheme, it's a dark but sparkly metallic gray, kinda like a darker chrome, but not as clear.
It can be found in most "Games Workshop" stores (uk/usa./netherlands).
Lastly, a few additional suggestions:
Since he's carrying that giga-blaster, why not add a sense of weight and balance witha pair of boots or leg-supports(they could mount on the backpack in truck mode.
That'd add to the gravitas (or failing that, to the gravity

).
As for his shoulders, some trucks have an oblong or semi circular cover on the roof, why not replace the stacks with 2 halfs of such that come together in truck mode?
I really want to see the finished form of this promising project, so I'm hoping my suggestions can contribute something to it..