Finally got the rust busted off the bottom rear bolts but the tops are killing me. I got it loose enough to spin but can't get into the space where I assume you need to get a wrench to keep the top nut from just spinning the rod and not removing the nut.

I'm assuming there is a nut below the frame you need to get a wrench on but I'm a big guy and can't squeeze in far enough to see it. Is it also an 18 mm like the top nut and the nut on the bottom bracket?

Any advice would be very much appreciated!!