If your 79+ hood prop spring just doesn't seem to want to hold your hood open enough, there are some possible causes.
1. The plastic bushings for the hood prop spring might be broken.
2. The prop tension might be out of adjustment - this is the metal bracket that is located on the prop spring just above the radiator support. Normally adjusting it as low as possible will provide the best result.
3. The channel in front of the radiator bracket (where the hood spring screws down) could be bent downward. You can remove the grille and put a bottle jack in between the lower support that runs horizontally behind the grille and the channel where the prop springs screws down. You'll see the reinforced sections from beneath the channel. Carefully bend each reinforced section back into its original position using the bottle jack. If your car has ever had any front end damage, this is the most likely cause.
One of my Spiders had a hood that just wouldn't open beyond about a 65 degree angle. I had swapped prop springs, replaced bushings, and tried everything I could to fix it. It had once had minor damage to the front edge of the hood (by the prior owner) which caused the channel to be bend downward. It was not apparent by looking at it. After bending it back, the hood now opens almost to a 90 degree angle after these fixes. It really makes working under the hood much easier!
End result looks like this:
