Well the eight and three quarter held up until I got it sold. I had quite a few "danger rides" around the neighborhood and over a dozen hard passes at the drag strip. I put the rear end on Craigslist letting people know I was keeping the brakes and the backing plate and the drums and all that for my 8 3/4 - 410 spool that I built and my phone was literally ringing off the hook for that 355 track lock.
So in conclusion I would say you're fine to get started, but work on that 8 3/4 or Dana 60 or whatever you're going to go with as quick as you can while you still got resale value in the eight and a quarter. It wouldn't hurt to service the old rear end before you put more strain on it with the new motor. You know just changing a couple quarts of oil.