2 speed rear end, 5 speed info needed

[HEI451]

I am in the planning stages of doing a 2 speed rear end for my 1964 Chevy C30 1 ton dually flatbed. I have the following questions for the board:

I presume the 2 speed rears were used on C40, 50, 60 series trucks?

Were they 1:1 and an over drive, or, under drive and 1:1?

The whole thing is to reduce the rpms of the big block 305E V6 I have in the truck on the highway. As is now, I have a very low gear single speed rear, and granny low 4 speed, with 1:1 top gear.

The next plan is to find a 5 speed with both granny low, and over drive top gear ratios, don't know if that exists, but asking might get answers I need.

The whole thing is to gain the best of two worlds, granny low torque pulling ability, and reduced rpm highway cruising at speed.

Thanks for any info.