This is REAL simple.
Stop overthinking it. You install the exact same length shaft that you have Now. I presume it's 25in.
because you keep talking about the "New" ['18,'19,20] year boats have a 30in shaft. And you think there is some kind of magic that your boat will preform better.
Nothing could be further from the truth.
If you've been around here long enough, then you have read the threads about raising your motor to optium height,
And you have a 25in shaft on "YOUR" boat......Now stick a 30in shaft on "YOUR" boat.
You will have a brand new motor that preforms Crappy. Why?
Because the shafts to long and you will run out of adjustment before you can get the motor high enough, causing a lot of drag.
So you will have a $20K- $25K investment and have a Slug! Oh.....and when you figure this out and say,
I need a 25in shaft.....The dealer will smile and rub their hands together.....It will be just like buying a new car and 2 months later decide you don't like it.
Obviously the ['18,'19' '20] boats have been redesigned somewhere, with a higher transom, so they need a 30in shaft.
I hope that helps you make the "Right" decision ?