Enjoi,
I am looking at this again while putting off plumbing repairs (I would rather work on trucks than houses). First, I am wondering what makes you think the truck is a 1966 model? I originally thought my 5500 was a 1965 model because that is the year given on the registration form. But it turns out that this truck is a 1966 model built and sold in late 1965. I am also wondering about your comment on the 6066 GMC Guy VIN decoder for large GMC trucks. I know it is not easy to follow, but the parts are mostly there. To start, I don't know what the leading DD stands for, but it is also on the top line of the tag for my 1964 1500 pickup, so I doubt that it is part of the VIN. When I look at the rest of your VIN number, according to the 6066 GMC Guy site, V = vacuum hydraulic brakes, 40 = model 4000, 11 = 120" wheelbase (this didn't work for my 5500), F = 1964 to 1966 model year, 23643 = number 13,642 off the assembly line, and E gives the GVW class (which I did not see described). The site also mentions that the older style cab was still available on conventional trucks in 1966, so this does not pin down the model year. However, I would be surprised to find a 305C engine in a 1966 truck. Good luck with your work on this.
