From the picture you've got a nice looking truck. My dad had a '69 Chevy C60 twin screw tandem axle (10 wheel) tractor with a 427 that we used until 1974. It was a good handling truck with a 5 spd. main and a 4 spd. aux. transmission. It would burn 8-10 more gallons of gas per day than the 409 equipped C80 dead axle tandem tractors that we were using at the same time. A twin screw truck drew more power from the motor since two differentials and a power divider were involved, thus more fuel was burned when compared with a dead axle tandem with only one differential and no power divider drawing power.
Your picture shows a single axle truck with dual wheels which would not be a twin screw, its a 6 wheel type. I noticed air horns on top, so you've got air brakes? That's a heavy type of a single axle C60 truck you've got, air brakes are a big plus in my opinion.
Looking forward to hearing more about your truck! James
