First off, very cool concept there RC. It seems to me though that your model is slightly out of proportion when looking at the reference. I think you need to scale down the top part of your bus (sorry.....'chickenmobile') because the roof is too high at the moment.
Also, this thing could seriously benefit from a new body kit in some areas, like nonclemente was saying. Such as maybe some intakes on the bonnet (or hood for all you yanks out there
) or even part of the engine sticking out the bonnet a la 'Fast and the Furious' style.