driveshaft length

[70 nova]

DOES ANYONE KNOW IF CHANGING FROM A 12 BOLT TO A 9 INCH IF THE DRIVESHAFT IS THE SAME LENGTH OR DOES IT HAVE TO BE LONGER OR SHORTER. I'M ASKING FOR A FRIEND WHO IS IN A TIME CRUNCH, AND TO KNOW THIS WOULD HELP HIM WITH IF HE NEEDS A NEW DRIVESHAFT OR NOT. THANKS.

[dadnova]

When I went to the Curry 9+ rear end I had to have a shaft built.
If he has the power to need the 9 inch he will also need the larger U joints.