Often a greasy job as its all the #,s seem to hand stamped but are almost 1/4" high and fairly deep.(my experience on 4 only).
As Tim stated, Never in one exact location- What fun! The #'s stamped are Most likely whats in the gears as it's just so much easier to change to whole third member as it is to just change the gears!. The factory would have the correct # even if optional ratio used.
The dealer may have done ?? if someone wanted a diff. ratio. Price?, replace the whole 3rd. member or replace the gears only.?.
Wrecking yard ratio change would almost guarantee a complete third member exchange as the price would dictate the 3rd. member vs. gears only change with labor and parts price to complete the change.
A stock 3.9 will work well for you in the 55-65 mph cruising range When you get used the the radiator fan noise!. Others have done it for Many miles!
Get it running and drive it some to see if whatever it has works for you,
Enjoy,
DJ