Car won't run past 60mph

I am not 100% on the gears the tag is missing. I also don't recall driving any faster than 70mph when the slant six was in there. I didn't have much seat time before pulling the motor. I guess there's possibility of a steep gear maybe something like 3.91's but in a /6 67' barracuda with a 7.25" rear? I better figure out what gears are in there this weekend. Whats the back yard mechanic way to determine the gears?