Gotta agree with Royce. This is a good idea. Especially the gear choice. It won't be a huge difference in power due to the gears, but the traction...sweet Jesus McGillicuddy!
Some more tips for a rear end with 30 plus year old axles about to be abused:
Check the axle shafts for trueness. A slightly out of kilter shaft is hard to tell with the eye, but can really exacerbate bearing problems when you do big burnouts, high speed runs, or hard launches. They can be checked with a dial indicator at almost any machine shop for, like, ten bucks.
OR... buy some new axles in the 31 spline variety from someone like Randy's ring and pinion or Currie (etc...) and your rear will be prepared for a brutal assault of torque. Yeah, I'm good at spending other people's money...
Change the bearings to high quality parts like BCA or something. The bearings are something often overlooked by lots of hotrodders. Since it'll be apart anyway, it won't hurt to replace them.
Change the gear oil to high quality stuff.
Nah, I'd go for the 3.25's too. You'll need a new gearset with the locker anyway. And factory gears are good, but the newer metalurgical processes used for gearmaking are way ahead of what they had back in '67. Might as well get what you want. And the lower gears will take a tick off of your e.t. at the strip. I just meant that in comparison to the increase in traction, the gears were secondary to that for perfomance.
Go for it man. And make sure whoever sets up your gearset knows their stuff. Even if you have to mail it to Royce.