have you thought of cutting the tops off so you keep them at 6ft? many plants will 'thicken' out when the tops are out. dont know if these will but worth a try.

wait and see if any one else posts a reply for you.

as long as your trees are healthy they will thicken up especially if you cut the tops off as SBG say's.
You dont say if it is an actual hedge that you want to improve the structure of or individual trees.
I have a row of leylandii that I 'stopped' at 6.5', it is now a solid hedge ( see my photo's. 12th pic down )

The problem wit a 'thick' Leylandii is that with poor air circulation they become vulnerable to the dreaded Aphid. Certainly cut the top but try to keep it reasonably thin.

chop them down and grow something better they are a pain