Those look like some J.C. Whitney-quality helper springs, not in any way "struts"
I'd remove the lower shock mount, so the helper has someplace to go, before loosening the lower spring mount.
For new springs? I'd Google "helper springs"
(How are the upper shock mounts? Broken yet?)
I'd be tempted to borrow a spring compressor from Kragen, or O'Reilleys, or whomever. Could also maybe use a bunch of wire ties.
I actually used zip ties to compress the front ones. Upper shock mounts look good, and already have the panel out of the way to access them. Sounds like removing them will be easy compared to getting them back on the car.
When searching most hits are the leaf spring helpers I mentioned in my original post. I have found some helper springs that look like they may work. I would have to measure the height of mine. I would have no idea on how to choose the "rate".