Fumblin,

Probably the most common conversion is sold by Patrick's. It will take you down to a 3:55 and supposedly allows you to run highway speeds at a comfortable RPM.

If I remember correctly, the ring and pinion run about $400.00. Another $300.00 for their installation kit.

A recent thread suggests that while it isn't real difficult, it should not be undertaken by your average backyard mechanic. There are also some special tools/equipment required.