Q. What type of ornamental conifer or spruce will thrive in soil that contains clay and holds moisture?
Thank you for your question.
Evergreen conifers to grow and thrive in clay soil. Well, there's three listed by Rutgers: Arborvitae, Juniper, and Yew. http://www.rutgersln.com/nursery/planting-for-specific-conditions/clay-tolerant-plants/
There are many Juniper varieties/cultivars available. The society has some pictured on their link page, have to goggle the rest for pictures.
Please contact the Missouri Extension, they wrote the book on helpful plant information. guides. http://extension.missouri.edu/main/DisplayCategory.aspx?C=40
Hope this helps.