Keeping in mind that I am a first time Vette owner, and for less than a year.............here is my dumb question:

I have noticed in pictures of other C3 bumper cars (as I have noticed on my own), that there seems to be a "sizable" gap at the top of the junction between the door and the front clip, especially when compared with the gap at the bottom.

Is this "normal"? Do the bodies just fit that badly?

Mine's an '81, so I can't say for sure about your year, but C1 - C3 vettes are well known for poor body fit.  In fact, if you restored one and went to the trouble to line everything up just perfectly, the NCRS (National Corvette Restorer's Society) at times has been known to deduct points when judging a car because no Vette ever left the factory with panels aligned that well.

Since the gap in question on your car is at the top of the hinge side of the door, I have to wonder if some of it isn't due to the door sagging over time?  If so, not too hard to rectify...

I understand that if the front radiator support has rust problems, the entire front end may sag, leaving a larger gap at the top of the door than at the bottom..

My '78 has a bigger gap at the top of the right door than the bottom, but an even gap with the left door. There are no rust issues that can be seen, so all I can attribute it to is poor factory fit.

Mine is terrible on the drivers side! I did everything I could when I had the body to rectify this to no availe. (Short of a different door.)

It's not uncommon for the gap to be larger at that point. Even something as simple as a hinge bushing can affect it. I'd start with the pin and bushing first before I attempted an adjustment.

I agree. It's something you look for when buying a C3, knowing that the gaps won't be even. Some can be adjusted, some can't.