how would someone set up some kind of equalent of single address space in longmode?

There are already a lot of topics on this board about setting up long mode and paging. If you don't want to wait too long for an answer you could try out the search.

However, depending upon how much detail you actually want the simple answer is just to map the paging as 1-to-1. You can't turn off paging in long mode so I doubt there is any other way.