I know it's gonna sound crazy but I experienced the same symptons on my '70 for awhile and even changed the ignition switch in the column.My problem was actually the nuetral safety switch connection on the transmission.Try squeezing the plug and re-installing or twisting it a little when plugged in.In order to solve my problem once and for all I had to trim a little bit of the rubber boot on the connector to get a better connection.Be sure to check the connection at the junction block on the firewall too.The fact that the car starts when jumping the solenoid would lead me to believe the ignition switch is good.

I wasted time and money replacing the ignition switch I didn't need and I'd hate for you to do it too!!! Let us know how you fixed it!!!