Driveshaft yoke question

I apologize if this is a repeated question. I'm on my phone so it is not easy to search.
I swapped from a 318/904/7.25 to 340/833/8.75 I am going to have the driveshaft shortened to fit. I have the correct slip yoke but I'm not sure what end my driveshaft needs on the rear end side. My 8.75 is a489 case and has sbp so does it have the 7290 yoke or the 7260 yoke. I am assuming the driveshaft is set up for a 7260 since it was a 7.25.
I saw on mop arts that the measurement is 2 1/8 for7260 & 2 5/8 for 7290 but where is that measured?

Thanks guys for the help


Justin