Please note something very important on Scneids455 GM illustration he posted on page 9. The top of the muffler shell should end up being even with the hanger (at the top). In other words, there looks to be no more than about 1.5" to 2.0" between the top pipe entering or leaving the muffler and the rubber isolator. Any more than this and then the front pipes hang too low and the tailpipes aim up in the air and too much of them show from the side. Also, with the muffler in the correct location, looking at it from the rear, the lowest portion should be about 1.0" above the sway bar. If it looks even or is below, the muffler is too low and the pipes will all be too low and incorrectly angled.
Look closely below at the photo of Bob's 70. It was set up according to the GM manual. You can see how the muffler hangs in relation to the sway bar and you can see how much of the tip shows below the quarter panel and what angle it is when the muffler is at the right height. Scheids455 illustration reveals a lot of useful information
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Hope this additional information helps. It is always hard to anticipate what things can occur until the task at hand is started isn't it?