Looking to upgrade my diff and gears to posi and something that will give me a little more power off the line. I currently have a 307 with a cam, lifters and headers and a 400 turbo trans. I have the original engine and plan to build that up in a year or so. But for now I'd like to do the rear end first. What do you guys suggest? I was thinking posi with 3.42 gears. I have a '71 Cutlass S.

 

3.42/3.36 are both good compromise for a little more pep but still driveable for hyway. Your best bet and a drop in would be a 403. Jmo.

 

A 403 would be extra horsepower and torque, for sure. If he doesn't run it hard because of the windowed block, it should last. A hundred extra cubic inches never hurts. "Anti Spin" (posi) would be a necessity with any gear ratio.

 

Thanks for the suggestions. A 403 would give me the power I am looking for.

 

Stroke the original 350 motor, 75 more ci. You can get an easy 400+ hp and 500+ ft/lbs of torque.