Looking at the information from the Chevelle site, early 10 bolt, '67-'69, Chevrolet only. It should be a standard u-joint used for years. BOP axles of from those years use a bigger cup on the u-joints and won't fit the Chevy, but you can buy a adaptor u-joint if you have a BOP driveline and Chevy axle.
Pull the cover off if you really can't decide on the year, the ring gear will have the date stamped on it along with the ratio.
The early cars from '64-'67 used a narrow rear axle, the bodys changed in '68 and used wider axles. Yours being narrow would fit the truck better and keep the tires under the fenders.
