Rocky,
There are several companies that market coilover shock absorbers, and the most affordable ones are either AVO or SPAX. You can order shocks from them complete with springs if required. For the front of a 600 chassis I would use a 250 lb. (115 kg) spring. I use a 200mm long spring with a 75mm spacer spring in the front. For the rear you will need VERY short coilover shocks that have a "pin type" mounting on both ends. I ended up having these custom made as I could not find anything short enough as a standard product. Spring rate should be somewhere around 350-380 lb (160-170 Kg). You can use a longer shock if you raise the top, rear shock mount. Usually this means welding in a piece of round tube with a top plate with a hole in it for the shock to mount.
The reverse eye leaf spring is an OK modification. You must be sure that it is done by someone who knows springs. There is some planning that must be done, if you are going to do this yourself. If the idea is to make a track racer, then you want a spring that only has a small "arch". The bigger the arch, the more camber change that will occur during wheel travel. You will want to put a transverse link in between the center bolt on the spring and one of the frame rails of the chassis. What this does is limit how much the center section of the leaf spring (between the two frame rails) can flex. It effectively makes the leaf spring into two 1/4 elliptical leaf springs and does stiffen the rate of the spring. It also helps in locating the spring and making sure that it stays centered. If you use the leaft spring, then you do not need coil springs. There were several Fiat 600 cars homologated with both a leaf spring and a coilover unit. In this arrangement the leaf would likely have been used as nothing more than a semi a-arm, and the total of the leaf and coil would have made up the rate of the spring system.
The amount of lowering that takes place will depend on how much you de-arch the leaf spring and the distance you move the eye at the end of the spring. If the eye is 25mm above the spring blade now and will be 25mm below the spring blade when the reconfiguration is done, then the front will be 50mm lower. Add to this the amount the spring was de-arched, and you have the total that the car was lowered. The person doing the de-arching should make sure that the overall width of the spring does not exceed the width of the standard spring (when fully compressed and flat) by more than 4mm per side, or the track on the car will be off. and the front will have too much negative camber. Finally, when lowering the car, and using standard shocks in the standard location, it is possible to lower the car so much that there is virtually NO travel left in the shocks. This can cause very severe handling problems, such as instant oversteer.
Hope this helps.
Regards
Pauyl Vanderheijden
www.scuderiatopolino.com