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There's a gearset that goes into the present speedo "socket". Unfortunately, to get the gears correct you need to do the inside one, too, which requires removing the tailpiece if I remember right. I think the set runs about $125.
Dallas, according to Marc Haibeck, you do not need to remove the tail piece. You remove the driveshaft and push the gear out the back with a coat hanger. I did just pay Marc $125 shipped for the correct gears for a 4.10 swap.