73 Shark gave these numbers to me a few years back for speedo gears... maybe they will help. Get the right one for stock tires, otherwise the speedo will be way off. Talking from experience, I am... but getting the wrong one somehow made my car about 15mph faster...

Here's old (circa 1976) P/Ns and teeth: 3987919/19 teeth, 3987918/18 teeth, 3987920/20 teeth, 3987922/22 teeth. Parts guy should be able to cross those to current numbers if they're still available.