The one thing with coil overs is that they can have a lack of travel. So in a daily driver - where you might tend to want to have a full tank of gas -- load up the trunk with luggage and then 4 people and off you go.... you can be bottoming out. I stayed with leaf springs on my new chassis in my '37 Ford for this very reason.
Coil overs are "tunable" for height (like plus or minus about 1/2" or so up and down) and most use them for ride height over the ride "quality". On cars that are going to be used to "handle" -- this is the system to go with. But Mary Pozzi was a National Champion auto crosser and her car used leaf springs!
My brother in laws '69 Camaro -- pro street -- is on leaf springs and I'm trying to get him to switch to coil overs -- it "cleans up" the rear end area and makes the car appear a little more 'modern' and we can then adjust ride height and get it really dialed in... but his car works just fine with leafs.
If your going to make your car a really nice handler - bring it up to 'date' - improve the resale - etc -- then coil overs would help in all of those areas... but I'm just saying that they aren't "required" to give you a good ride or handling. SPRINGS really just hold the car up... and leaf springs hold the rear end into position (side to side loads). It's really more important to get these springs "right" for the weight and use regardless of whether or not they're coils or leafs. It's just easier with coils.
Look at SpeedTech - or DSE - or BMR - or other manufacturers - they all have kits for conversions etc. Then it's a matter of budget and what you want to do.
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