A modern mechanical cam and springs requires what work on the heads?

In most cases, with mild cams under .550 lift, you can find a single spring that will work fine, as long as you aren't planning on a full race or competition engine. That's one area where I actually like Comp. They have several different single springs that will work on many different applications. As long as you find one that has a similar seat pressure, open pressure and spring rate, you can use it.