All GM car rears made before 1972 (when GM dropped Spicer in favor of American Axel Works) used a Spicer 1310 universal joint. So your 12 bolt should have a 1310 joint.
That doesn't mean some one couldn't have changed out a 10 bolt for a 12 bolt out of a light truck in the past that did use a 1330 U-joint. I hope not as a truck 12 bolt is weak compared to a car 12 bolt, because the truck rear wasn't built for racing but carrying weight. Because of this you have a small pinion and fewer gear choices, and the axles have huge bearings (with smaller OD axles) to carry weight.
Here is information provided by one of this boards sponsors, CARiD:
You can use the inside caliper (the pointy part) to measure your main cap at the edge of the parting line to determine the diameter of your you joint.
Note the reference to a tab being present on your yoke as to whether you need an inside snap ring universal or not.