You'll need to lower the front and rear end proportionally to maintain the balance/handling of the bike. Typically this is done with a spacer in the shock to restrict the travel and tubing in the fork to limit their travel or shortening the forks internals followed by revalving. This is common practice with super moto bikes so perhaps do some web searches to get a better feel for what's involved. Any good suspension shop will be able to help you out and unless you are well versed in suspension maintenance I'd leave it to the pros. I know that MX Tech has performs this modification for others.
If you feel you are getting close now with the technique you are using and only need to lower the bike a bit more you might consider removing some material from the seat foam. Most people will use a belt sander to cut down the material and make the profile of the seat narrower.
If you search this forum using the term lower you will come up with lots of other suggestions including using different length dog bones etc as well. Lots of info there.
Hope that helps,
David