First off, I too am not a fan of stop leaks as they tend to gum up the entire system. That said, XJ heater cores are probably one of the worst engineering designs of all time. You have to weigh the fact that using stop leak may eventually lead to water pump failure or overheating issues down the road. As far as the double sided felt tape goes, what fun is owning an XJ if it doesn't squeak or rattle? That's how I know mine is still running! Well that and having to turn my radio up loud enough to hear over the rattles, squeaks and road noise.
I have used Bar's stop leak (http://barsproducts.com/) in the past and it did stop the leak. Never kept the vehicle long enough to see if other problems developed.