Were you stepping on the brake when you were trying to start it? If so, it may have been a short in the brake light circuit all along. Try unpluging the wire harness from the stop light switch and then step on the brakes. I'm not sure where 63 Chev trucks had fuses. The older ones were on the firewall, but I think they had moved them by 63. It probably uses an old SFE type fuse which is easily diagnosed good or bad. Maybe someone else can help with the location of the stop light fuse. There is a good chance you have a short from the stoplight switch on back, but you have to eliminate one thing at a time.
