Sorry to be the bearer of bad news, but there is little chance of you being able to recover the captive nut as the magnet trick has so many other sections of steel to be attracted to....
There is a way to replace it but it does mean you will have to open up the A pillar from the back and weld a plate to the inside of the pillar. I dont drill the plate until after I have the door aligned...I usually use the remaining existing captive nuts and bolts, align the door, tighten the existing bolts....then remove the door leaving the top hinge in place on the body.
Drill and tap the blank plate through the hinge body and install the bolt...replace the door.
Monday morning quarterbacking I know, but sad experiences have taught me to tap out all mounting points very early in the cab build process...then install old bolts in the holes prior to prime and paint etc...
This link may give you an idea on the process...
http://67-72chevytrucks.com/vboard/s...=438045&page=5
Hope this helps...
