Rear End Question(s)

Here is a direct fit and it is a stronger sure grip
check ii out, I would love to have the sure grip out of it for my girl, but I don't know if the splines would be the same on my 66 7 1/4
http://www.forabodiesonly.com/mopar/showthread.php?t=155545
So are you saying they made a 7-1/4" suregrip as well? Or did I interpret that wrong... If so how can you tell if your own isn't a sure grip Too.