Harrison Ford, even after Star Wars, was not the first choice for Indiana Jones. Instead, Tom Selleck was picked, and you can see his screen test on the fourth DVD in the Indiana Jones boxed set. I mean the special features disc, not that pap Lucas and Spielberg attempted to foist upon us last year. Anyway, Selleck dropped out in order to appear in Magnum, P.I., Ford was Indy, and all was right in the world.
On a somewhat related note, an episode of Quantum Leap was planned in which Sam would leap into Magnum. That didn't happen either.