You appear to have two problems.
First, you are using the wrong driven gear. That's why they are being chewed up. Hopefully you haven't ruined the drive gear, as that will require tranny tailshaft removal to fix.
Remove the driven gear, and hold it in your hand, with the tip pointing up. Now look at the teeth. Do they curve up and to the right, or the left? Whichever you see, you need the other. That's what they mean when they say right hand or left hand gear.
Second, you need to alter the number of teeth to correct the speedo. It's a simple percentage calculation. If you are seeing 15 under at, say, 60 mph, you're off by 25 percent. You need to add teeth to the driven gear to slow the speedo, remove teeth to speed it up.
Here's where I have bought gears before and had good luck:
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4
You can use a portable GPS to help calibrate your speedo. Works like a charm!
Doug