How much of a gap should there be between the top of the fender(rear part) and the front of the A pillar. I have seen no gap and have seen about a sixteenth of an inch. This is on a 67 GTO so it should be the same on a Chevelle. A picture would be helpful.

We (My son and I) own a 67 Chevelle that's getting a Frame-Off restore. Someone else may be able to give you factory specs,(I doubt it.) but what I can tell you is to align the gaps as best to fit the car.
Go to car shows. If you pay close attention, that particular gap is never consistent from car to car (66-67's), even on the best of restorations.

Heck, I watched an episode of OverHaulin' once wherein they did a 67 Chevelle and you wouldn't believe how un-Godly WIDE the gap was between the fender and bottom of the windshield post.

Like I said, I'd align it where it best fits the car. If all your other gaps are nice fitting, it won't be an issue (Unless its wide enough to stick your hand through like the OverHaulin car).

The width of that gap can only get so small before the hood wont close between the fenders, or your fender to door gap is too small. I went through the same thought process on my 69 until I realized I couldn't make the gap any smaller or my hood wouldn't close. In the end I have maybe 3/8's of and inch, and it looks good.

Front to back on the fender gets controlled by the door, side to side on the fender gets controlled by the hood. Sometimes you split the difference depending on how much adjustability you have in each panel, but it's a good rule of thumb.

my 66
the driver side fender was up against the Apillar cover
passenger side was over 1/4" gap.

Also---fenders will stand outside of the doors by up to an 1/8"

Right now I have both fenders up against the pillar covers. The reason I am asking is that I don't ever remember seeing a gap there but at recent car shows in my area I am seeing a gap in that area.
With it up against the cover I have perfect gaps all around my doors. The only area that it is a little narrow is at the top of the door and if I brought the fender forward it would open up that gap a little but also open up the lower part of the door also.