That's a "Big Cab" GMC COE. Better known as a, "Cannonball" COE. Very popular, & sought after truck. A 660 would be a 5 1/2 ton. I had a '50 model GMC "Cannonball" 600 series 5 ton. Mine had a 318ci. GMC gas engine. That truck will weigh about 14000lb. Compared to a 2 ton 350, 3 ton 450, & 3 1/2 ton 470 series "Small Cab" COE, this trucks cab is 70"in. wide compared to 58"in. wide, on the small cab COE's. Cab height will be around 82"in., compared to a 350 COE at 74"in., & a 450, & 470, at 77"in. Wheelbase width will be around 73"in., compared to 61"in., for the 350 series , & 67"in., for the 450, & 470 series. Fender to fender width is 91"in., compared to 73"in., for the 350 series, & 80"in., for the 450, & 470 series COE's. Hope this wasn't too confusing. You can see the 600 series COE, I had here. It's the primer brown, & red one.
http://www.classiccoe.com/oldcoetrucks/canball.htm
