Its been in the tech articles for years...heres the link and the post article itself.
http://www.67-72chevytrucks.com/tech/fenders.html
you want it easy...
69 to 72 Chev fenders are unto themselves.
67 fenders are the same for GMC or Chev with no marker lights.
a 68 Chev and 68-72 GMC fenders are the same with marker light holes.
Here's the tech post anyway.
Fender Interchangeability 67-72 Chevy & GMC Pickups
By Rod
Here's the real low down on the fenders. 1967 fenders are interchangeable between GMC or Chevrolet this if you want fenders without marker lights. 1968 Fenders are interchangeable between GMC and Chevrolet but theses will have marker light holes. Here's where the difference is. 1968-1972 GMC fenders are all the same and will fit any such year. As I said before you can put a 68 Chev fender on a GMC and it is identical to the 68-72 GMC fender. A 67 Chev fender would fit any year GMC 67-72 but you would lose the marker light hole. Chevrolet Fenders are interchangeable between 1969-1972 They are very different from GMC fenders of this era. Chevrolet fenders are recessed at the front of the fender to accept the full grill from 1969-1972 Chevrolet grill. Mind you it will bolt on to a GMC rad cradle but you would not be able to put on the Eyebrow piece on the Fender that GMC's have. It looks rather stupid this way. For you would likely have the GMC top bar across the hood and a recessed area for the Chev Grill. And not be able to mount the fender eyebrow pieces typical of the 69-72 GMC. 1967 and 1968 GMC and Chev Fenders Eyebrows are the same and are for 67 and 68 Chev and GMC trucks. Then only 69-72 GMC fender Eyebrows will only fit a GMC fender. You can put the 67-68 trim on a GMC 69-72 fender but not on a 69-72 Chev fender.
Rod