Here is a skinny armed redneck way to break the nut loose…
Get the six point socket, extension and breaker bar on the nut.
Get a cable come-along attached to the three point hitch structure of the mower.
Arrange the wrench and come-along so they are about the same height for a fairly level pull on the cable.
Arrange the wrench and come-along so the wrench is about 90 degrees to the pull point.
Double the cable so you get twice the pull and half the distance for each handle stroke.
Hook the come-along’s hook onto the wrench handle and then put some safety wire holding it on somewhat. I also recommend putting a vise grip pliers on both sides of the hook to keep the hook in place.
Then tighten the come-along to move the extension against the side of the hole. You can put some wood or brass between the extension and hole to prevent minor damage if you want or if the mower deck is thin.
Then get the cable fairly tight and put yourself out of harm’s way with a pipe extension on the handle of the come-along. I also set up a 4x4 post guard if possible or I think of it or I feel nervous about the tension that is building up in the come-along.
Be sure you rigged to pull the correct direction to loosen the nut.
Tighten cable and watch the magic happen.
Honestly when I did this it popped loose after a few clicks on the come-along ratchet and was not very dangerous (but ignorance may have been bliss?). The hole in my deck was at the 3 o’clock position (if noon is at front center of the mower, under the three point hitch) so in a good position to use the 3PH structure as the pulling point.
I did have a 3/4 breaker bar that was at least 3 feet long but no pipe extension to use. I don’t like to struggle with things so when I couldn’t get it loose by arm strength I resorted to the pulling method.
And since you have good access to the nuts you can spray some penetrating oil on the threads. I use PB Blaster since it is easy to purchase and has worked well for me.