Speedo gear install and Gear reduction box??

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.

See http://www.gaugeguys.com/ratioadapt.htm for more info.

You don't HAVE to get that box -- but it will be more accurate. Especially since in 90 the internal gear in the tranny changed which makes it harder to compensate for lower gears like 3.73's. You can just get a gear and put it in as well -- just know your speedo isn't 100% right on (if it ever was!).