If you want to take it right down to the new growth I can see near the bottom, you can, but there are a couple of 'rules' or best practice to observe - do it in early June, and saw the trunk through at an angle to allow rain to run off. Take it down as low as you like so long as you're not damaging the growth lower down - if you leave a stretch of trunk, it will put out shoots from that as well as forcing more from the base though.

What a giant!