65 C10 Rear End Question

[Daron Craig]

My truck has the 12 bolt rear end in it. I'm not 100% sure what ratio. I do know that when I spin the driver side wheel one way the passenger side wheel spins the opposite way. I planned on pulling the cover and changing the fluid so I can count the teeth. If it turns out to have at least a 3.5 something gear in it, what would be a good option for making it a posi? Locker? Weld the spider gears? Any info or pictures would be greatly appreciated. Thanks,