drive shaft

[cigar-]

My next large undertaking will be converting my truck from a three speed to an automatic. I've assembled the cross member and other parts, including a decent steering column. Now I'm told (but not sure if its for certain) that the drive shaft is different between a three speed truck and an automatic truck.

Not being a transmission mechanic, I want to make sure before I begin pulling things off the truck. Can anyone enlighten me about this topic? Are the drive shaft lengths different for a three speed versus and automatic? And......what is there (if any) in difference between a drive shaft for a Turbo 350 and a Turbo 400 transmission? I believe the Turbo 350 and 400 transmissions are the same exact length (based on my hot rod experiences).