conexion between manifold and ducts : at what level?

I have seen in the new book a new way to do the manifold, with a piece of barrel instead of the ash-tray.
This sort of dome looked a bit difficult, but I had planned to burry quite a lot the heater,
and I wanted to know if the duct could start vertical with an elbow...

But with this new way, that looks great, the duct starts directly from the bell!

Are there different posibilities for the height of the start of the duct?
What are the rules for this?

I have a ceiling height problem, so I have to bury a little more than normal the heater.
And I go out under the floor.

This is the real most important question I hve to solve and dicide right now!!!
Thanks for your attention and answers.