8.75 Diff Questions 2nd try

SOrry for the title of my fist post. I have a 8.75 Rear end not a 8.25

Ok, I have been restoring/ COnverting a 74 Duster from a 904 with 7.25 rearend to a 833 with a 8.75 rear end. I bought a rearend off Craigs list and it look pretty good to me but as it was latter found, the diff has some issues, at least I think it is the diff. The rearend came with new upgraded axels and green bearings already installed on the axels. The guy I bought iot from is a hobbiest and told me the whole system, hosing, diff and axles were ready to be assembled and installed. It is installed and running but I have a growl from the rearend at 30-60 mpg and it gets louder when I let off the gas and hold in the clutch. It will make nopise almost to the point when the car stops. It sounds like a bunch of rocks boucing around in the case. No when I looked at the diff before installing it, I noted no issues with it. I talked to a local diff shop and they think it has an issue with the pinion bearings. I have rebuilt my 727 in my Road Runner and have been a Aviation Mechanic for a long time, so delicate and complicated devices don't scare me but I think a diff is a demanding build. I have a 489 case and need a bebuild kit. I also have a 741 case that was going to go into the Road Runner. Which one is better if any, and has anyone built these up. Can anyone give me some pointers. Thanks./