Is there an easy way to determine my gears? would like to install overdrive transmis

[327-C10]

I would like to determine my 68 C-10, 1/2 ton's rear gears. Aside from opening it up and counting teeth is there an easyer way to tell the gearing.

Any chance any of the codes on the inside of the glove box doors show the ratio. The truck is a 307, 3 speed.

Has anyone tried one of these overdrive (manual) 5 speeds. What is the preferred rear gears with this type of set up. When I'm done I would like it to be a daily driver good enough for the Friday night cruises.

Thanks in advance for any comments.