Having my 73 Sat do the same thing check the ignition switch connection to the bulk head wiring there is a 10-12 gauge wire that i will bet is corroded swelled up or green.jump past the connection Thats not all go from there to the main wiring loom mine had a broken wire about 8-10 inches in from the same switch. I also have had to replace the one half of the fuse box with one of the cheapees from advance do to lack of connections and everything now works every time.
By the way that button is a seat belt bypass not belted in no start