I just got a 67 chevelle. It's been in the shop for a few weeks...fixing all the little problems here and there. I would estimate it to have maybe between 350 and 400 horsepower. The guy I bought it from said that it could be fast, but the rear end would need to be upgraded, since it is still using the original from when the car used to have a 283. Would a Ford 9" be my best option? Would it make the difference in performance? How much would the parts cost to upgrade my rear end? Any help is appreciated.

You may have to include more info about what you currently have in the car before anyone can suggest "upgrades" for it. I'm guessing you have a big block engine to make 350 to 400 hp. The type of transmission you have would be helpful. If you can find rear axel codes (numbers stamped on or near the differential housing, perhaps some of the members can help you identify what you already have for gears and can offer advice for improving your setup.

I've got a chevy 350, and a TH350 tranny. Right now, I don't have access to the rearend, because it's in the shop...

I would stick with a Gm 12 bolt.