I've got the same problem
Mine is a 1990 w/351. After it rains at night, she wouldn't start the following morning. I checked and she was getting spark. I sprayed starter fluid in the intake and she fired right up. This happens everytime after a hard rain and I can start her everytime with the starter fluid. It has something to do with my fuel injection system. When she starts properly, you can hear the fuel pump run for a couple seconds and then shut off. After the rain, I can hear the fuel pump run and run and never shut off. She won't start without the starter fluid unless the pump cuts off. Next time listen to see if the fuel pump is just running and running. Any idea what's happening here?