ok, what size do I need??? is it an 8.5???

I guess I should tell you its in a 69 SS396, and its a 12 bolt....lol

Nope, not 8.5". You need a posi unit made for a 12-bolt rearend. They're 8.875" or something close to that.
The ring gear is .375" larger than in an 8.5" rear. Am I correct folks?

Joe is right you need a 12 bolt. You have to get it to fit your gear. A 3.08-3.73 series 3 , 4.10and lower series 4. 8.5" ring gear is a 10 bolt rear.

Find out for sure which rear end you have and what the gear ratio is and send me a email. I'll fix you up with one.