I'm going to ask: Why do you think you need a 9"? The 8" will handle pretty much anything a streetable 289 can throw at it and will consume less power than a 9 inch, not to mention it's lighter as well. The 8" is good for 450ish horsepower, more if you upgrade the internals. Currie makes everything for the 8" as well as the 9 inch.

 

I had a 66 fairlane and broke an axle like it was a toothpick and i only had about 300 rwhp....AND that was on street tires messing around in town one night...i plan on making around 350 naturally aspirated then adding nitrous to get it around 450 rwhp...dont forget that nitrous makes alot of torque!!! I plan on doing the axle swap last and getting the power first...it maybe cheaper to just build the 8 inch but i didnt know there were alot of parts out there for it....with a built 8 inch, how much power can it handle? Do they make a detroit locker for it?

 

Another option is to go with a wider 9" rear out of a 67-68 model. That 2" can be used for wheels with more backspacing and ultimately wider wheels.

 

Yes, you can get a Detroit for an 8 inch. I'm running a similar power number on a stock 8" and have no problems with 3.80:1 gears. I don't race anymore, but still go through the gears every once in a while (okay...everytime I drive it).

I'm sure I could break it if I really beat on the car, but I'm sure I could break a 9 inch too.