Run a jumper lead directly from a known good battery voltage like the starter end of the + cable to the + coil terminal and try to start it. If it starts quicker, track down the source of the voltage loss through the ignition switch circuit. A 55 International I fixed several years ago with your exact symptoms had been to every shop in town, repeatedly, and was dropping over 1 volt at a dirty, loose connection on the back of the ignition switch. After that fix, the truck owner was convinced I could walk on water!
