talk about big try growing a 12-20 footer. lol
Why?
Regarding the plants going over your fence.
Only one thing will keep them under 6 feet if they are already six feet tall.
Radical LST, forcing every branch down to under four feet.
Supercropping is possible, even on larger branches, with experience.
At this point, I'd force the plants to my will, and discard any branches that don't survive the ordeal.
I did exactly this with two plants in my greenhouse, today, because they had reached the ceiling. Supercropped about forty tops back to four feet on top of screening.
I'd love to let my plants grow naturally, but circumstances force me to keep them under 9 feet.