Coilovers are not a "bolt on" for a '56 Chevy. Heidt uses Aldan coilovers on his custom units. In other words, it's going to take some fabrication work. Don't know if Heidt would be willing to work with you, but Aldan should be cooperative. There must be someone who posts here who's done up a tri-five with coilovers.
Incidentally, I've redone that page on my blog (dealing with front spring rates) and it's now much more user friendly.