7 1/4 to 8 3/4 conversion

First off a Merry Christmas and a happy New Year to you all!

I am in the process of putting a 8 3/4 in my 64 Valiant to replace the 7 1/4 that has finally given up the ghost. The question is how long to make the drive shaft. I have seen a difference of 2.26" between the length of the two, but I'm wondering if this is good enough measurement to have the the drive shaft shortened by. I will not be able to get under the car to take a tape to it till spring, and would like to get the shaft ready to go before then. Thanks in advance for any help you can give.