As it turns out, SCS has already showcased a steampunk truck and i think that pretty much sums up my ideas of what steampunk is.

Click for full size.
This definitely doesn't mean you shouldn't continue work on your own. I just think that image shows exactly what i expected to see when you said steampunk. Frames around the sleeper window, locker like artwork on the storage unit on the side, nuts and bolts running across the bumper, air vents and all kinds of details.
I am sure your steampunk paintjob will turn out great either way!