My 2 cents worth:

My 3:55 runs about 3000-3200 rpm at 70 mph. TH400. Auburn Pro-Posi. 450+ HP. 500+ lbs Torque.

If its the original rear end, there should be a 2 letter designation stamped on rear side of the driver's side tube. This will tell you what the original rear gear was. Take a peak there.

If you do the work yourself, be sure to get an installation kit. This includes bearings, shims, etc. Also, when you order your parts, make sure your sales rep understands that you have a "BOP 8.2" rear end. Sounds simple enough, but... You're looking at about $700 worth of parts, including the posi unit.