6.1 is Euro 6. You will have an ad-blue filler underneath the diesel filler.
On mine, I put a cill tank in from CAK tanks, about 27L. There is also space for tanks at the rear quarter- CAK do one, but there was a thread on here with a poster who had a much better solution, and avoided dealing with the vagueness of CAK.
I have a heater fitted by driver’s step, but this space is also available if not.