The open space is from the shape of the left FX thigh. It's actually larger at the bottom than at the top. This can be remedied with trimming of course and eliminate the barrel legged look. I made my left match the size of the right leg, and I think I trimmed a wedge that measured about an inch off the top and about 3 inches off the bottom.
As to the AP helmet. I have a really big noggin as well, and it's a tight fit. One thing you can do is to trim off about 1/4 inch all the way around the neck opening. This little bit will give you a couple more inches to work with (remember HS geometry), but you may want to ask AP for a longer piece of neck trim when you order in case you have to do this.
Good luck.