How do you measure your rear-end to know what size you have 8" or 9"? and I currently have is what I believe is a stock rear-end of 3.0 ( 289/4bl/Auto), and would to replace with it 3.53 or 3.73? not sure, but I would like a punch at the rear and just not a cruise automatic type, would replace the ring and pin work?
How much would something like this cost, I found a set at summitt racing ( basic set ) for 159.00
Thanks for all the help, and would it be worth adding a transgo shift kitto the transmission?

 

Hey Kenny.....welcome to our world. I bet you feel like the red headed step child since nobody has answered your question. If you have a C or Acode 289 you will have an 8" rear. Only the K code cars got the 9". However, just to ease your mind, I want to introduce you to the FAQ we have on here. Scroll down to the differential section and read all about the rears in our ponies.
https://mustangforums.com/m_3027145/tm.htm