I have seen that truck in person. It's actually a mix of various parts, namely the aforementioned '48 cab, inline Atlas 5, and S10 chassis. It had a bed from a '54/'55 1rst, as well as some Corvette parts, namely wheels and brake components.
I still think it's odd that GM went with a 5 cylinder. So far every article I've seen for stock Colorado and the people that i know that have them say it's underpowered. Seems it would have been better to just use the 4.2 inline 6.
