Very rare to find one that big without some rot in the center. Might try to exploit that, but it can present a problem as well. I'd take as many wedges as you have and make a few gluts (I think is the term, a wooden wedge).
The secret to splitting big wood is exploit it's inherent weaknesses. I know this sounds crazy, but I have split a lot of trees in my life, for firewood and bow wood. Look for a natural check that parts the pith line in the end, and follow it. If not start one that centers the pith line and try to tie into an existing check.
You can beat on a log the rest of your life if you put your wedge in the wrong places. And you don't have pound the wedges with a ton of force. When you are starting the wedge especially, use a very light blow until you get the wedge to set, or grip. If not it will bounce back out whatever amount you manage to drive it in on each blow. I use a very sharp, very narrow hatchet head to start a split. Once you set a wedge, let the wood pop and settle in for a minute or two, give it time to part as far as it will.
Follow the grain, and plan for that before you start. Try to only have one split, so be careful where you start it on the very first blow. Two or three false starts can screw up the whole thing. If you start your split right alongside an existing check or other split the wood will tear out along the grain and form "stringers" joining the two. Essentially you'll be pushing two splits in front of your wedges.
Sometimes the big, big stuff ain't as good as you'd hope. But you never know. Pictures would be outstanding.
