So I pulled the wheel off today, and removed the factory bump stop. 13mm deep socket, no problem.
I measured the flat space on the control arm and there is a 3 in diameter area to work with. The landing spot for the bump stop on the frame is a bit larger, it is rectangularish and is about 3 1/2 by 3 3/4 in.
IMPORTANT - These control arms may not be nissan factory. I had replaced them a few years back due to the bushings being rusted/frozen. I think they are nissan or dorman, but not 100% sure. If you are considering this, look at the pics of mine, and look at yours. If yours are different, do what I did. Remove your bump stop and measure what you have to work with!
Then I found this option. SumoSprings SSF-106-40. 500 lb rating (rated per axle) at 50% compression. I think that is a good option to firm the rear up a bit when under load like when my travel trailer is hitched up. But should not make the right harsh since it is the softest of sumosprings. And the size appears to be right at 3 inch diameter, 4 inches tall. I have about 1 3/4 inches of lift in the rear and there was about 4 1/2 inches to work with - rough measurement unloaded between the control arm and the frame where the bump stop lands.
Tell me, anything I am missing as to why this won't work? Any reason this is a really bad idea? Wouldn't be my first - LOL.