OK I think I may have found my answer in this thread, https://www.gmforum.com/t281877/

Looks like there is a second bolt that I might have missed that allows you to remove the heater pipe. Tanks again for you help. Hopefully I can get this done soon.

Good find, it seems that there are very few issues that have not been covered on this forum, especially on the older vehicles. Using the right search terms can make a huge difference. Thats why it is important to post any issues that you come across because there are a lot of folks who are reading and learning. I know I have learned a lot just from browsing. If you can, toss in a few pictures of the problem and possibly the solution.