Driveshaft length.....again.

Hi all,
I am replacing my 71/4 with a 8 1/4 3.55SG and need to know the correct length from u joint to u joint. 67 Barracuda.....727 to 8 1/4. I realize this topic has been beaten to death but someone HAS to have this info. I just can't find it........maybe someone with a 340 Duster/demon has this info

Thanks,Mike