Actually, they were probably figuring $60/ hour with no more than 2 hours. I think my first shorty took me about 3 hours!

Actually, the best way to do it is to drill out the rivits holding the end cap on, or take out the screws if you can. Then you can usually see what's going on inside. If you go too far, you could mess up the internal baffles. Usually, I try to cut the outer skin and pull that off so that I am taking apart the can one layer at a time. With my 636, it was all welded, so I ended up chiselling it off.

Once you get to the length you want, then put the end cap back on, re-drill, and install new rivits...remount and be over joyed in saving a butt ton of money from taking it to a shop!!

Granted, it will probalby take you 3-5 hours of your life including removing and installing the can...but fuggit!!