First, the crank bolt has normal threads. Lefty loosey, righty tighty.
Try spraying some penetrant fluid on the bolt and let it go for a couple of minutes. Either get a better impact gun, or a better air supply line. Impact tools are very sensitive to air volume, just like turbo exhaust systems. The more air that gets to the gun, the better it will work. If you haven't ever tried, put a pressure gauge at the end of the line just before the gun and hold it wide open. Most lines will see a significant drop in working air pressure at gun. I wasn't convinced until I did this, and found that my 120psi static pressure was all the way down to 60psi at the gun. A bigger line and high flow fittings brought it up to 100psi and my Snap On IM6100 became the most valued impact in the shop...
Short of all that, get a bigger lever. Make sure you have the flywheel locked in place. Put your 18" breaker bar on, and use a big pipe over it if you have to. It will come loose, but keep in mind that your breaker bar isn't very large when it comes to crank bolts. Example, newer Prelude bolts call for 180+ ft-lbs...
