I just re-read your post and what you are referring to and what J's and MCR make is a Final Drive Ratio for the diff. The Final Gear Ratio in the gearbox is different. It is a fairly complex piece. A company called ATS (I think) made one quite early on.

What I am refering to is the Final Drive Ratio which is the crown and pinion gear in the diff. OEM is 4:1. There are 4.3, 4.4, 4.56 and 4.77 options (and possibly more).

http://www.bulletproofautomotive.com...ail.php?ID=837

http://www.bulletproofautomotive.com...il.php?ID=4241