[Big Dave]

I think he can do it. Chevy trucks are light by pickup standards. The 6.5 is highly under-rated (although it really does not compete against the Cummins or Powerstroke). Especially if equipped with 6.2 pre-combustion chambers.

I didn't get a good read of the truck's configuration.70 MPH and 2,200 RPM in third gear indicates the very low axle ratio one would expect of a Bonneville car. Probably a T-56 manual. One would expect a nice, slick belly pan on a Bonneville vehicle.

There are stories out of Detroit where in the 80s, A Suburban was fitted up with a 6.2 NA, a 700R4 transmission, and 2.73 gears and was good for 30 MPG. It would not out-accelerate a Benz 240D.

One could go one of two ways with a truck of this vintage. Either a 6.5 GM or a 4BT3.9 Cummins. They weigh about the same and make about the same nominal power. The Cummins has more potential (it is a 12 valve Cummin with two cylinders missing) but the GM is probably smotther. Both are fairly easy to come by.