The factory didn’t deliver 4 speed cars with the 8 3/4. However with increasing warranty claims of broken 7 1/4s on 4 speed cars the dealers received guidance to replace all the warranty claims with the 8.75. I would imagine as a customer you could walk into the dealer with a warranty claim and ask if they would replace the 7.25 with a 8.75. The 4 speed cars came with 8.75 rears in 66 while automatic cars still got the 7.25. Through the petitions of racers especially Junior Stock NHRA recognized this change and allowed racers change over to 8.75s regardless of power train. This subject has come up numerous times on classracer.com.