Removing the insulation is a tedious, messy job, which is why so many people skip it. There are many theories on this, ranging from dry ice to solvents, but I found the best was to soften the insulation with a propane torch and scrape it off with a gasket scraper. The remaining adhesive residue was just as difficult to remove. I used a variety of solvents ranging from acetone to MEK. Make sure you do this in a well-ventilated area!