yea thats the best reason to tear that rear apart that you'll ever have.
all you have to do it take the center section out and count the teeth i assume it is a 9" rear.
ex. 31 ring gear teeth X 10 pinion teeth = 310 rear ratio
if i had to guess i would say it either has a 350 or 355 ratio. but counting the teeth is best way to tell.
sometimes you can tape a string to your driveshaft on the bottom and mark the ground at the bottom of your wheel and pull ahead till your wheel makes one complete revolution and then go by how many times the string wraps the driveshaft but if it wraps 3 1/2 times you have to flip a coin to decide whether it is 350 or 355 for real.
i say thats only a ball park way to measure it.
hope this helps a little bit.