I'm getting ready to start a 3 or 4 day road trip, but thought I would post this now, and hopefully someone can point me in the right direction by the time I make it back home.

I did a tranny swap on my '69 C-20 last year...put in the A-833/MY6 OD and it works great
except that the speedo gear in the tranny gives me a false reading on the gauge. How do I know, you may ask? Well, @ 2200 rpms the speedo needle points at 100 MPH. I KNOW I'm not going that fast,probably in the 55-60 MPH range with my 4:56
rear gears.(It sure looks cool,though)

Is there a link to a good resource for purchasing the correct gear for my tranny so that I can get an accurate reading on the speedo gauge? I'm just not good at finding this kind of info on my own.Thanks,

Try these...
http://www.bgsoflex.com/speedo1.html or
http://www.hotrodders.com/forum/speedometer-information-60786.html

here is another: http://www.transmissioncenter.net/speedometer_calibration_______va.htm
Besdes having all the necessary gears for just about anything they also have ratio reducers and ratio increasers to cover what cannot be had with a simple driven gear change.

One of the easiest ways I've found to figure out the error all by youself is to find a length of road with Measured Mile signs. Note the 1/10 odometer number at the start of the measured mile and again at the end. Simple enough to figure out if your odometer is turning faster or slower and by how much. Like if it reads 1.2 miles then it is 20% too fast as an example. Adjust your cable driven ratio by that much. The beauty of this is it works at any speed. A mile is a mile whether you drive it at 20 or 60.

Interstate highway mileposts are another good place to check your speedo. Most are posted every 1/10 or 2/10 of a mile. Driving 10 miles on the interstate for the error check will really give you an accurate reading.

My own personal experience was that each tooth would make a 5 MPH difference. Take the cable off the trans and on the inside of the fitting is the driven gear. Count the teeth on it and once you know if you are over or under, you can figure out the required tooth count from that.

If all you did was change the tranny, then all you need to do is look at what gear was used in the old tranny (assuming it was correct to begin with) and change your current tranny gear accordingly.
To figure out what the number of teeth on the output shaft worm, just look at the end of it and count the number of times the teeth intersect the face of it.

Another quick and dirty way to check the actual speed these days is with a GPS unit. If you don't have one, ask around...any of the aftermarket car Nav units will give you a very accurate speed in MPH or Knots (if you are so inclined!)