Just thought I'd update because I did remove the furnace today (Suburban GT3). I did not need to remove the sink. I sprayed some corrosion inhibitor into the space between the two pipes to help them separate, and then tapped (firmly!) on the back of the furnace unit with a hammer.
The pipes started to separate right away, but I had to keep tapping for the whole 8" or so that the pipes overlapped. There was only light
surface-type rust, but it was a friction fit that had been in place for 30+ years. I spent about ten minutes today getting it out (thanks to the info in this thread, I went in with a plan
If I can only find a burner for it (have not been able to find one
), I will reinstall it (or a whole 'nother Suburban GT3, if anyone has one they would like to sell).