When I went from a 3.54 to a 3.58 diff I found a company in Denver what can make up a gear reduction box in virtually any ratio you want. The box itself is small, see the picture in the link below. You send them your speedometer cable marked where you want the box placed. They cut the cable, attach the parts to connect each half of the cable to the box and send it back. You get a cable and box assembly back that is the same length as the cable originally was. They also called me once the had the cable to verify the gearing I needed. I got the completed assembly back about 9 days after I sent it.
My converter box is located on the speedometer cable so that it lays on top of the transmission. The Speedometer is dead steady and accfurate according to my GPS. Total cost for everything including the tips and ferrules for the cut cable and shipping was $168.53
The company is
Deluxe Speedometer and Cable Service
2330 S. Kalamath St.
Unit A
Denver Colorado
888-907-6505
www.deluxeSpeedometer.com
I was quite happy with my customer experience. NAYYY, just a happy customer.
EDIT: A link to my installation.