First check all of the grounds. Then start with a meter at the batteries and work back until you loose voltage. When you loose it. you have found the source of your troubles.If you have power at the relay on the fender well, try jumping the small terminal to the battery side of the relay see if it spins the starter. If it does, then check the safety switches and ignition switch. If it doesn't then check the relay itself. There are also some fusible links hooked to the batt. side of the relay that might make you pull your hair out. Your trouble is going to be somewhere in one or more of those spots. Don't let it scare you. The transmission came out and went back one bolt at a time, take this the same way, one step at a time. I've had to start over at the beginning many times because I missed something. If you don't find it start again. It always turns out to be some stupid little thing you overlooked. I've been doing things like this for 40 years, electrical troubleshooting can be a bear, but only if you let it. You had to have taken the starter out. Is it hooked up correctly? The wire coming off of the starter side of the relay excites the starter solenoid, the battery cable goes directly to the starter.