Make sure you have the right spring, there is a left and a right.
There's a socket/tool to get to turn the spring:

Here's the tool in action, in this pic the hinge is in the closed position; it will need to be in the open position for this to work:

for removal: You put the tool on the spring and twist is away from the hinge, then you tap the other end of the spring towards the end the tool is on until the spring will clear the hinge when you release the tension. Then let the tesion off and you're done. The tool must be used on the bottom of the hinge spring because you can't tap it the other way because of how it hooks into the hinge.
This pic shows the end the tool must be used on, the square end of the spring, the tool won't work on the rounded end.

for installation: hook the square end in the hinge and then put tension on the spring and once it clears the hinge tap it back down (Make sure you don't tap the other end so far it comes loose or you'll learn some new dirty words

) and relese the tesion and you're done: The last pic is of a LH UPPER hinge, just for reference. Having a helper would do great too because you need two hands to work the spring tool and the helper to tap the spring for you.
The last one i did, was on the truck, with the door off, but it was a PITA. If you're not worried about the paint around it, it's not so bad. If you have great paint and want to save it, it almost certainly has to come off the truck alltogether.
I don't know where to get the tool, My dad bought that one before i was born, and gave it to me when he gave me his old '77.
Good luck!