Look at 12-o'clock and 6-o'clock.
These two nuts fasten the head to the cylinder, only.
The other four are on the rocker arm towers, as you already know.
Tighten them in a criss-cross pattern, much like putting a wheel on a car.
They are to be tightened in 3 stages. It is in the owners handbook (underseat size), if you have one (you should).
From Sue's extensive thread titled Condition of LRB's Valves!!!!, Reply #48:
[size=14]"13. Torque up the nuts in steps just like our owners manual says. (11, 18 then final 25 foot pounds) DO NOT EXCEED 25 ft. lbs.!!"[/size]