What cam should I get?

Well, stock, there should be a tag bolted on the rear end with the ratio on it.
However, I have been there, like you, and did not have a tag. So, I errored on the side of caution and did a gear oil change since who knows when the last time that cover was off, if ever. The ratio as you may allready know is stamped on the ring gear.

The driveshaft method is not to accurate, but close enuff to get close enuff to make a good stab at it.