I will add a link to the Help Page and fine tune this entry a bit later:
Within the same model year, the 62 Series and deVille Series fenders will be the same.
Within the same model year the Fleetwood 60 Special and Eldorado front fenders will be similar in overall shape, however you have different trim mounting holes for the side trim found on the 62 series and deVilles vs the wheel well, lower rocker and lower fender script found on the Fleetwood and Eldo. Of course holes can be filled in and new ones drilled so these fenders are easy to internchange.
Within the same model year the 75 Series and Commercial Chassis front fenders are the same however, they are very different from all other body styles due to the 1959 to 1965 center Fleetwood, and 59 to 64 Commercial Chassis center body sections they are adapted to. That is the shape at the rear is different.
The 63 & 64 Fenders are considerably different especially at the very front. First, a more subtle difference is the larger squared raised area near the side front for the wide side molding found on the 63's. The 64 side trim remained thin to the very front where it joined the thinner eyebrow molding.
The big difference is in the physical structure of the front of the fenders. the 64 had a large fender extension and headlight pan that bolted into the front of the fender, whereas the 63 had a have moon shape lower section the wide eyebrow trim attached to and a smaller headlight pan, that really wasn't an integral part of the fender like the 64 extension. Another main outward physical difference is going to be where they butt up nest to the upper grill. If you look at the illustrations of the front of the 63 vs the 64 you will see they are quite different, especially where they meet the fender. There are a few other subtle differences but these are the biggies.
See the 63 headlight and eyebrow pans:
I have attached a picture below of a 63 and 64 fender side by side.
Here are the 63 and 64 front Illustrations. Note the different shapes between the fenders, especially where they butt up next to the upper grill: