The single speed rear ratio will be 6.17. A two speed rear has 6.13 or 8.10. I have much the same truck and after a lot of thought I went for a later 6 cyl petrol straight 6 and a five speed. This lets the truck run about as fast as you would ever want to go on stock suspension and brakes. Its nice at 55 and at 60 the limiting factor is the suspension and the violence of the bumps. The driver needs to have no back or spine problems.
As everyone here says above- get a new platform if you want to go any faster, but thats just not the same and the classic truck is gone forever. Better yet, Resto-mod it, update the minimum, sit back and enjoy the ride.
