The GMC's during the '60's/70's were a more HD type of truck, therefore, they had parallel leaf springs on the rear in lieu of coils. GM built Chevy's and GMC's at the Fremont CA plant. The GMC's of the mid '60's had the 303 V6 engine in lieu of a V8.
I can only guess that the original fabrication company of my truck had a contract to supply a number of specially modified medium duty trucks, some of which were spec'd to be dually's. When it comes to licensing the cost is based on the gross unladen weight, ie; a C20 is cheaper than a C30.
To meet the contract obligations some trucks/chassis were ordered as GMC's, some as Chevys. In the truck world it is quite common to purchase what is called "Slider's", complete trucks minus the rear differentials. Many years ago I had a motor home that had a 454 V8, dual rear wheels with a dual wheel tag axle. The RV was rated at 26,k GVW. by the RV manufacture, yet the chassis was actually a C20, Suburban chassis that had been modified for the air bagged dual wheels and tag axle. I did not keep the RV for very long, I found it to be very unsafe.
It is pretty well an established fact that GM did not put the 454 V8's in anything larger than a C30, the large HD trucks have 427 tall block, that being GM's HD truck engine.
My truck is still titled and licensed as a C20 with the unladen weight of a C20. Wm.
Last edited by Wm L; 09/10/2021 3:42 PM.
