42 GMC Rear End Selection

I pulled the torque tube from my truck and would like to replace it with an open driveline with a better gear ratio. I'll be running a GMC 270 with an SM420. I would LOVE to keep it 6-lug, but i'm flexible to keep the cost down. I have a rear end from a 59 chevy truck with 3:90's, but thought maybe i could do better than that ratio since the goal is a comfy 70mph with a little power of acceleration left in it. I've got the whole truck dissassembled right now, and need to work back to front in order to roll it araound while working. I don't want to make the wrong decision now and not even realize it until a year from now when it's running! Suggestions? Guidance?

 

Here's a nifty online calculator to help choose tire size and gear ratios:

http://www.csgnetwork.com/multirpmcalc.html

 

The 59 rear may be too wide for the 42. Four wheel drive s-10s have 6-lugs that might work, and they have some pretty decent ratios.

 

There are also several makes of Japanese trucks that use the same 6 x 5 1/2 bolt pattern that might provide an appropriate width and ratio rear axle assembly.

Ray