Suspension upgrade on a 68 barracuda

The rear leafspring on my car are extremely worn and they sagging ( 3-5") a lot. I'm looking to replace them. has anyone put rear coil over conversion on. how difficult is it but also looking for the best deal for the best price and I'll might go back factory let me know what you all think I'm open to any ideas but I do have a budget. And what do I need to buy it is a 68 barracuda notchback original 225 slant 63 on the tree but now has a 318 with a 8 1/4 rear end