I am looking to buy a coil over kit for my 240Z. Does anyone know any relevant web addresses I can check out? Has any one ever done this mod?

Arizona Z Car Has them...

Ground Control also has some...

You may be able to get some parts from some of the other suppliers, even some from stock car magazines that may be cheaper... but the complete kits are all there with good directions on how to put them in.

You will have to cut your strut towers down and remove the stock spring perch. It is not a hard job, just take your time. Make sure not to burn thru the towers when you weld them together as you won't be able to get the strut inserts in if the weld blob goes thru.