Negatives are great, but not necessary. Just extend the heavy 5 rep weights for a few weeks. You can train with the heavy weights for weeks on end. If you start feeling burned out or your progress is completely standing still, then SD and start over.
6 weeks is too short for my taste. I believe in extending the hypertrophy for as long as possible before restarting. Of course I am a max-stimmer and we use 12 week cycles with some seriously heavy loads near the end. Dan Moore (HST expert and Max-stim author) has told me it could be very beneficial to keep going once I am maxing out and reaching my strength limits. In other words he thinks it may be superior to go for a few weeks at the same weights once you reach your strength limit, rather than Sd early and possibly miss out on some growth.
Oh, and Bryan also has said this, that there is nothing wrong with extending the heavier weights for more than two weeks, that it could be very beneficial. 6 to 8 weeks was never written in stone by Haycock, that is just the basic HST routine, which of course can be personalized depending on your goals, etc.