At this point I'd pull the bell and tranny and get them apart out of the car. I have a C shaped wrench that I use that came in some cheap a$$ tool kit I bought, but yours seems way to tight for that to work.
Just an FYI- an impact will loose alot of its force going thru swivels and extensions. My 1/2 drive IR won't take my drive shaft off because I had to use a bunch of them to get around the larger diameter of the aluminum driveshaft. I ended up pulling the U joint apart so I didn't need anything but the socket and a quick tap of the trigger and they bolts were on the floor.