There isn't a speedo gear that will correct for 3.55s and up for anything after 1989. In 1990 Ford went to an 8 tooth drive gear, the most teeth any Ford driven gear had was 21 which will only correct for 3.27s. If you have a 1989 or earlier they had a 7 tooth drive gear which allows you to correct for up to 3.73s with the 21 tooth gear. OR you can pull the tailhshaft and swap the driven gear to a 6 or 7 tooth for whatever combo you want so keep that all in mind if you want an accurate speedo.
And plz don't mention the 23 tooth gear, it is a Chrysler mesh and will get eaten up in a Ford trans. Ask me how I know.
At any rate, I would go with 4.10s for a NA street car. Highway RPM with the .68 overdrive and 4.10s is not bad at all unless you like going 80 all the time and even then. 3.55s to 3.73s won't be a huge difference though.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.