dave, i have a 9" out of a mustang. i run 33 spline axels. had to use there bearings and pull the stock ones out. i think they said i could use the stock bearings with the 31. the 33's were the biggest i could use with a stock housing. the reason i went with the 33's was because at the time there was no warranty on the 28 or 31 and there was a 10 year warranty on the 33'sand up. i think they may have changed that now.