Normally a 79 with a 403 has a th350 transmission, so that's why I mentioned the colors earlier, thinking you had a th350. But it looks like you have a th400 transmission.
The drive gear is the large rearward steel gear. Trying to count the teeth from that view isn't possible, since that's not the way to count them. You have to count them face on, like in your second view of the driven gear. For that you'd have to remove the tail housing so you can look dead on at the face and count the teeth around it.
Hopefully you won't have to change the drive gear, since it's more work than a th350 drive gear. There are probably videos on youtube how to do it. But maybe you can find a driven gear that will work with your drive gear and correct your speedo. But, I think you'll have to take off the tail housing and count the drive gear teeth to know which way to go with the driven gear number of teeth, and see what range of driven gear tooth counts will work with your drive gear.
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Steve F.
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