I thought you could only disregard those bits of the runway you didn't need. That is to say if the landing distance doesn't take you into the midzone or end.
That sounds a bit short for a '74. but I suppose as long as you can keep it straight, that's OK.
If you're doing a CAT ll in the first place the assumption must be that the RVRs are similar to the touchdown.
In other words, I don't know. Tell us another!