To be honest Elley, my first thought when I read your first post was the same as Quillman. A morticing machine or drill press morticing attachment would be the best. However, you would have to mount it in an open window and hang the boards outside to do 80". Still, those are really deep mortices. If I were going to try and do that with a router I would try and have almost all the waste wood removed and just do a light finish cut with the router and probably in stages of depth. This isn't a particularly easy task.
I would tend to go at least partly like Stick is suggesting and use a table saw. You can easily make a deep groove with a table saw and tenons are easily done with a straight router bit to almost any length you want them.
Have a good look at the pictures in Stick's link. It shows in one picture glue being applied only to the middle of the assembly. The table is going to expand and contract across it's width with seasonal humidity changes so you have to allow it to move or you will get cracked boards and joints. I really like the idea of the dovetail in the one picture but it is a much more difficult job.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.