Drive shaft length ???

Ok Guys , Here goes

I have done a search on this forumn and have gotten so many different opinions that its really confusing.

This is what I have : 1973 Duster, 8 1/4 rear end, 904 trans, 360 engine

I am putting in a 8 3/4 rear end, 741 casing.

Is there too many variables in this equation to tell me what I need to do with my current driveshaft. Do I need to shorten it and if so how much ?

I would like to get it shortened if needed before hand so it would be an easy swap.

What do you guys think. Is there one answer out there that every body agrees on or do I just have to be patient and measure it when I get the new rear end in ?

Bob