Firstly, I would try and avoid long portions of text in a VR environment!
Head up display style text which is fixed regardless of orientation has its place and so does text that is fixed in space, and it very much depends on the purpose of the text.
You mention about the possible need to look away from the text. Again - this depends on context. Might the user need to look away, or is the text such that you really need the user to focus on reading the text, and you don't want them to look away anyway.
I would not scroll when looking down because the act of moving the head down is likely to have some associated eye movement downwards. If the user is looking down in order to see the text further down, and the text moves up there's likely to be on overcompensating factor in the relative motion of the head, the eyes, and the scrolling text. Next thing you know, the user's vomiting on the carpet.
A quick nod downwards to trigger an autoscroll may work, perhaps with an opposing upwards flick (whatever the opposite of a nod is) in order to stop scrolling if desired.
Obviously no scrolling is needed if it fits in the view, but you may need to be careful about not making the text fill too much of the view as it can be tiring to read if it results in a lot of eye movement, which may be the case for HUD style placement.
Paginated text changing with a side flick of the head is another option.
Unfortunately, I'm inclined to say, that there's too much case of 'it depends'. It depends on what the general environment is; what the text needs to convey; the relevance of the text in the local environment at the time it's displayed; what else might be distracting the user at the time; ...
We also don't know if your specific environment has hand motion sensing. I mention small head movements above, but hand movements are equally valid if they can be associated with the text as opposed to anything else going on in the environment.
One option to consider is that you just have the text read out loud.
I was involved in VR in the 90's but that was a long time ago...