What can an 8" Rearend handle?How about a 9"?

Traction is more important than torque/HP.

A stock conventional 9" with crappy tires would handle 500hp no problem. But a locked 9" running slicks is a whole other story.

Same goes for the 8"

Are you planning on running an open/conventional axle? How about slicks?

I'd say an 8" open rear end running typical street tires is probably good up until about 250 RWHP.

supposedly the axles are the weak link in both the 8" and 9" assuming you have 28 splines. However, I have broken 3 8" differentials and haven't phased a 28 spline axle yet. Traction, torque, and shock are the main contributors to failure. A 9" was the logical solution for me and I haven't had any problems. When you consider that Ford never used a 8" in anything with more than 225 hp, there shouldn't be much to question. With a 8", durability will be the deciding factor, you can baby it around all the time and granny shift to keep the rear from grenading or you can go ahead and upgrade to a 9" and never have to worry about it.

I tend to agree with what's been posted.
Traction will kill an 8" in an instant.
No traction will give it a slow death over about 250 rwhp.
I have to disagree on the axles though. I have broken 3 or so 8" rears, but it is always the diff or gears that broke for me. The axles are the same 28 splines that are in the 9". The 68-up carrier is fairly strong. Your 67 might have the old 8" carrier, no vertical ribs, it is supposed to be weak.
Shorter gears will also weaken your 8". The engine has more torque multiplication and the gear is weaker the lower you go.
I found it very easy to break an 8" with 4.11s, but I have had complete destruction with a 3.00 as well... Just a guess, I was running around 325 rwhp through a toploader.
I think by the time you buy all the parts to beef up an 8", you could build a very mild 9". The 9" parts are everywhere and reasonably priced, the 8" stuff requires some looking and costs more because they are not nearly as popular as the 9".
Make sense?
Dave

you see my setup in the sig I would be lucky if Iam pushing 300hp but some how I blew up my spider gears. In my 9inch that had 3.25 in it. So I put a locker in and a set of 3.73's in. Iam very happy now.