I looked at an earlier set of answers that seem to apply to this situation, but they are not quite the same and the question is different. I have a large pin that is threaded on one end. It is 1" dia and about 3" long. It holds a very large blade onto a stump jumper on a brush cutter. I worked very diligently with a 1-5/8" 3/4" breaker bar with a 5' long cheater bar in an attempt to remove them on the machine. I think I applied a torque of about 700 lbs-ft just pulling on it and about 1000 lbs-ft by putting my whole weight on it and trying to 'jerk' it to bust it loose. I was unable to do so and decided to remove the entire assembly to take to my shop. In the shop, I have applied WD-40 again (as I had for several days in the field) and used a 1000 # air wrench to try to get it off.
Finally I applied over several hours a 1/3 part acetone to 2/3 transmission oil to both nuts. After over night, I still was unable to break the nuts loose so I heated one with a torch to almost red (I could see the color from the heat). After waiting an hour for it to cool down, I was able to get it off with the air wrench (1-5/8" socket). So I tried again on the other one. I could not get it off, so I heated it as I had the first one. After some more bashing with the air wrench, it too came off.
I was told that I could just use a large drift to drive these bolts out of the bar material that they are mounted through - nope. I tried that, no effect. So I put one of the nuts two-thirds back on the bolt. I hit it with a hammer using a 2x4 for protection. Broke the 2x4 after a couple of whacks. Finally I went after it with the hammer directly with just another piece of metal between, then with a 5 lb sledge - no dice.
Now I'm stuck, I used a press arrangement to jack down on the bolts with the nut part way installed. I applied about 3000# directly to each of the nuts with the bolt head above the ground - neither bolt will budge. I then heated the area around the bolt with an acetylene torch, trying to not heat up the bolt. I was unable to get it very hot, but it was quite hot - the bolts are still tight.
The bolts are flat headed (2" in dia head) with a 1-1/2" dia x 5/8" high step for the blade. The shank of the bolt is 1" with all of that 1" part out of the material on the opposite side from the blade. The part of the bolt's shank that is in the other material appears to be larger about 1-1/4" thick. It might be keyed, that is, with a swaged portion about a half inch long and 3/16" thick that goes into a notch in the material. Cannot tell if the bolt has turned, but it will not budge when I try to turn it with a pipe wrench.
The material the bolt goes through is 1" thick with another 1/4" plate around the hole. There was a lot of corrosion in the hole which I dug out and attempted to break loose using a screwdriver and the acetone/transmission fluid mix (ran out of WD-40 anyway). Then I heated the bolt head and shank from one side and the other so that they were both red-hot (dull red) so now I am about to bash on that thing with hammer and if it won't come out, I'll cut it into pieces with the torch and drive it out, and pay $30 apiece for new bolts and nuts. One way or another I'll get it out of there. I don't want to do that because the identical bolts might not be easily obtained. I have found something that looks similar, but I cannot tell for sure.
Question is - what should I have done when all else failed?
BTW my experience here suggests that the heating of the nut was essential to its removal.