Figuring Out U-Joint Needed

[cebra]

Continuing to work out some bugs on my truck as it is near complete. I have driveshaft vibrations going on and when I put a new rearend in, I noticed my u-joint that attaches to the rearend was too narrow (did not go from tang to tang on rearend). The truck was a 350/350 long bed with 2-piece driveshaft and 12 bolt rearend. It now has a LS and a new 1-piece driveshaft. It will need a hybrid u-joint; I can pull the u-joint and get the x measurements (cap and width to fit driveshaft) but how do I get the cap and width measurements for the y to fit my rearend? Was a standard u-joint used on all 12 bolt rearends? Can I measure ID tang to tang to get the y width, if so how do I get Y cap measurement? Thanks for any guidance.