Looks like you have a 10 bolt chevy rear.
To check what ratio you have there are 2 ways.
If it has a posi you can just mark a tire or brake drum and turn the pinion by hand untill you get a full rotation at the tire/drum and count how many turns you had to turn the pinion.
For a open rear you have to hold one of the tires from spinning and mark the other tire/drum and turn the pinion by hand counting the rotations of the pinion to get a full rotation at the tire/drum take that number and multiply by 2.
This is to take in account of the gear reduction in the spider and side gears.
OR you can just pull the cover and there should be a stamping on the ring gear. If not you can just count the teeth on the pinion and then on the ring gear and just do the math.