You can't. The gear to fix the speedometer doesn't exist. The closest one will still leave your speedometer off by a noticeable amount, and it's a bit fragile. You either need to get a Speedcal to electronically adjust the output from the VSS, or you can have the tranny taken apart and have the driven gear changed. Then you can buy a gear for the VSS to match your 4.10's. Unless you're looking to rebuild your tranny soon, your best bet is the Speedcal.
Oh, and Welcome to StangNet!