I would use a large easy out and adjustable wrench to remove it. Hardware stores have them. Here is a set on eBay
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4650756252

You will need to know approximately how large the inside hole is so that you will know what size easy out to purchase. You might could bring the old muffler with you to find the right size easy out.

I've seen people use 5 foot long pipes with a easy out to remove parts if the parts are frozen up.