First thing to do is trick I picked up from someone on this board; Take a hammer and hit the end of the cylinder that is over the threaded portion of the gland evenly all the way around the cylinder. Don't hit it hard enough to dent it, but do hit it with some authority, go around several times. Then try your wrench again, I bet it comes right off.
I don't have a spanner, I have a homemade wrench. I torched out of 1/4 flat a large y looking piece, and drilled holes in the end of it. I built this to bolt to a 7.3 diesel waterpump pulley to hold it while I took another homemade wrench to break loose the radiator fan, which is threaded to the snoot of the waterpump.
Looking around for a way to take these glands loose, I took my homemade y and put one 1/4-20 bolt with a nut in just one of the holes. I stick it in the hole in the gland and let the rest of the y rub against the chrome cylinder rod. It has broke everyone of them loose so far, I have done 4 different cylinders on my Case 480c. But I do have to hit it with a hammer all the way around or they will not come loose.