Lugnutz has a good writeup on picking a rear end ratio. For the I6, you might want to be revving pretty high because you need to keep it in its power band. These trucks are bricks, so you need a decent amount of power to make them cruise at 70.

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1958 Task Force Truck
"Frank" gets a new lease on life
Follow in the DITY Gallery
1959 3200 Task Force
The Ballad of Enkidu
The Saga in the DITY Gallery ~ and the story continues