[71GMC_3/4T]

If you're trying to loosen the bolts, use one of the other bolts to hold the flywheel. You'll end up with the other one tightened, but you'll have the first one out. If you work them enough, you'll finally be able to get all of them out this way.
Also, when you're wrenching, try to have the handle of your breakover bar passing through the center of the flywheel. This gives you advantage over the bolt.
I helped a buddy do a flex plate on a ferd van and we were able to get it this way without impact tools. Couldn't have gotten an impact in there anyway since we only pulled the tranny back far enough to get to the bolts. We used head bolts to slide the tranny back and still keep it close enough not to have to unhook everything else except the driveshaft.