After going from a 10 bolt to a 12 bolt in my 70 chevelle my drive shaft needs to be shortned.What is the best way to measure to do this so I don't make a mistake and have it to short or to long. Thanks for any input.

http://www.dennysdriveshaft.com/how_to_measure.html

Actually, it would be better to ask the shop who will do the shortening how THEY would prefer you measure things.