Assuming a stock ratio, is there an easy way to determine the stock ratio (i.e. build sheet, stamping on the axle, etc.)

Thanks!

It should give you the ratio on the option lable, which is either in the glove box or the inner fender (if it's still there). I would guess 3.73, but I may be way off.

There is also a stamping on the axle tube. I helped my friend find it on his 77 K5 because the sticker was missing in his glove box. It was located on the driver side axle tube, but I can't remember exactly where. If I remember right it was a 3 letter code. Be prepared to scrape off dirt and stuff too. Then call the parts department (I think it was the parts department, my friend made the call) at the dealership and they should be able to tell you what that code means.