Driveline Vibration

Hi guys, new to this site and new to Mopars, I have a 70 340 Duster with a 340/727/8.75 rear. At speeds above 30mph I get a very bad drive line vibration. I got underneath and looked for play in the u joints, they seemed fine but I found a lot of play (.187 in play) where the drive shaft slides into the transmission. So I have the following questions.
-Can the tailshaft bushing be replaced without the transmission being pulled.
-Could it be possible that the drive shaft is to short?
-How much of the yoke should project into the transmission?
-How long is a stock drive shaft for this car?
-What is normal for play between the spline and yoke?

Great web site, Thanks for your help in advance...... Jack

ps: This car was a four speed converted to the auto, could that be a factor?