The criteria for choosing the diameter of a drive shaft doesn't include whether or not it will hit something. You must first choose the right shaft based on very important criteria. If,after choosing the correct shaft,you find that it hits something,you must move that something. Mild steel is more than strong enough,but you chose chrome moly instead. Chrome moly won't harm anything or cause any problems,but it will cost more money. Same thing with the pinion yoke. The possible problem that might bite you in the future is the diameter of the shaft that you chose. The most critical aspect of a drive shaft is its critical speed. The critical speed of a drive shaft is the speed at which the natural frequency of the shaft intersects with the rotational frequency of the shaft. The forces get multiplied suddenly and cause the drive shaft to act like a jump rope.
When your shaft breaks during a launch at the drag strip,it's because of weak material. When it breaks at high speed,it's because you've hit the critical speed of the drive shaft. You'll probably never go fast enough to hit this speed in a quarter mile race,but you might in a mile.
The critical speed of your 3" drive shaft,at the length that the typical turbo Buick requires,is about 6,350 rpm. It's almost identical to its mild steel counterpart,so you gained no critical speed by choosing the chrome moly. You will hit that speed at around 150 mph. If you would have chosen a 3.5" diameter shaft,your critical speed would have risen dramatically. This is why Mr.Spool advised you to purchase a 3.5" shaft in post #7. Don't ever approach 150 mph with this shaft.