After many, many years being involved in the Chevy/GMC restoration hobby a fact surfaced about these trucks. I had never paid attention to the center tall “dip” in the firewall.
One day, trying to create a new reproduction rubber floor mat, I said to myself, “Why is this rounded area protruding into where a new floor must be? “ Its existence added so much more investment for our company to develop the correct floor mat tooling. A flat firewall would have cost half the price in tooling.
Surprise! Looking at the dip from the engine compartment side with a 235 six cylinder engine in place, the answer was evident. This long engine requires this “dip” or it can not fit in a 1955-59 GM truck.
Without it, and keeping a six cylinder, the wheelbase of these trucks would have required at least another 4 inches in length. Not a good idea for parking in a standard home garage. Turning a sharp corner would be a problem with the wider radius now required. The drive shaft would need an additional length.
Engineers eliminated this by bringing the long six cylinder engine deeper into the cab by adding the “dip”. Of course, the shorter V-8 engine did not require the “dip” but all cabs were made the same. And now you know – if you ever had a curiosity.
Last comment: On the assembly line the cabs were lowered on the chassis with the engines attached to the frame rails. This larger “dip” helps prevent the six cylinder from accidentally touching the firewall while being lowered in place!
