Yes, leave the metal shell in. But I don't believe heat is the best / easiest way to remove the bushings. Here's what I do:
Drill several small holes through the rubber part of the old bushing. Don't damage the metal shell.
Attach a vise grip to the inner metal core bushing, and rotate it out. It should come out easy if you weakened the rubber enough with the drill.
Now you're left with the outer metal shell, and lot's of residual rubber. To get out the residual rubber, just use a circular wire brush attachment for your electric drill (experiment with various shapes / sizes for best results) The wire brush will eat through the rubber and leave the metal shell perfectly smooth, making it easy to install the poly bushings. Took me about 20-30 minutes for both sides.