All you need is new leaf springs and the correct shackles.
Also, from your other post, I noticed you car doesn't have an engine in it up front, and half your engine parts (including the heads) are in the trunk. So the car is sitting nose high with a ton of weight in the trunk. Not saying you don't need new leaf springs, but there's a lot more load on them now than there would be if you car was at the proper height and the weight was distributed like normal.
Too many people just figure that the old suspension is no good and needs to be tossed for coilovers. The old suspension parts do need to be replaced because they're worn out, but nothing is wrong with the design for the vast majority of applications that people use these cars for. Converting to coilovers in the back means a full redesign, usually a 3 or 4 link system. Most of those kits cost $1,500 or more. Or you could spend $300 on new leaf springs and bushings, which will work just fine for probably 99% of the things most people use these cars for.