You definitely should put a loop around it... It'll keep the DS from hitting the ground and getting thrown through the rear seats of gas tank, or nailing the car behind you if it does come apart.
If the DS was well built it should have decent balance and vibration shouldn't be a huge issue... the issue is more noise and harmonics, IMO. There are plenty of calculators online for calculating the critical speeds; Where the shaft reaches a peak stress level (This does NOT mean it's going to fail at that speed!). It varies quite a bit, but for the MKIII DS's I have calculated for you don't want to do the majority of driving on it doing 70MPH to 90MPH. So, you'd be well advised to get a lightweight two piece DS if you commute a long distance every day and average 80MPH.
Now, with the warnings and doomsday-er BS out of the way: I've been running a steel DS for years without any problems or major swelling in the center. Just crawl under the car and check it for and signs of fractures and measure it for any significant change in diameter from time to time (like whenever you change your oil).
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