I'm rebuilding a '72 GS455, which now has a replaced 12-bolt rear (2-something gear ratio) and no driveshaft. Everything else is pretty much original, but the previous Owner had a mishap with the driveline... I looked into the manual, but did not find a spec of what size the original rear end has been (except that the ratio has been 3.08 with the TH400, non Stage 1). Neither is there a spec for the lenght of the driveshaft (I notice that there are at least two different lengths for sale, 54 1/2" and 55 1/2"). => So should I start to look for a 10-bolt or a 12-bolt rear end, and what size of ring gear? => What difference in the driveline defines whether it uses the 54 1/2" or 55 1/2" driveshaft? Any other recommendations will be much appreciated too! Have for example considered whether to rebuild the current 12-bolt to have 3.42 gears, or to change the complete rear end. I plan to do a mild street/strip rebuild, but try to keep the look of the car close to the original.
The original rear in the 72 Skylark/GS would have been an 8.5 10 bolt. That rear was plenty strong, and had bolt in axles. Buick went to the 8.5 from the 8.2 that was used in 1968-70. The 68-70 Buick 8.2 had no aftermarket support, but the 8.5 uses a GM corporate center section, and aftermarket support is great. The driveshaft is 1/2-3/4" shorter when going from an 8.2 to an 8.5, and the 12 bolt I believe. The 12 bolt requires a special U-joint with 2 different cup sizes. The 8.5 uses the same U-joint as the 8.2. If you can find a Buick 71-72 8.5 10 bolt, use it. They are great rears, I have a 1971 8.5 built with 30 spline Moser axles (28 spline was stock), and Eaton Posi.
Get a 8.5 rear and driveshaft from a 71 or 72 Buick or Oldsmobile A body and you will be in business as they are plenty strong and have parts o' plenty.
Thank you for the good info! Think I'd better start with finding the 8.5 rear end first. Propably the shorter length driveshaft is the one I need, but better to measure to be on the safe side.
