my understanding is that the fenders are minutely specific to each of the following types:
- 70-73
- 74-75
- 1976
- 77-78
- 79-81
Now in addition to that above list, you will read people here get into the minutia of original versus replacement stamping variants, but I am NOT going to touch on this;
With those above types, it all comes down to the presence or absence of bracketry to align the bumper cover or lower valance, and bolt holes drilled to align with the header panel or bumper cover with the fender's edge.
BUT(!)
Just about any (OEM) fender can be made to work for any year, so accordingly, you can make an aftermarket fender work as well.
For the 70-73 cars, you will probably
want to attach the correct valance bracket on each fender, but you can get away without them...
I too have read that the aftermarket parts are generally regarded as inferior to OE stampings - but my opinion is that a great deal of the remaining NOS sheet metal is stuff that was flawed and returned - so you might be dealing with similar issues with an NOS part versus a new AMD part...
I know I have pulled fenders off of 73, 74, 76, and many 79-81 cars to know that the forward edges are indeed unique to them all;
It is possible that 77-78 were shared with 76, but I cannot say with clarity, because I don't recall pulling 77/78 fenders off;
I believe that the 76 was unique in that the (?)dense foam/rubber bumper cover needed a discernable "lip" on the forward edge of the fender to hold the edge of the bumper cover in place - and I do recall that at least 77 bumper covers were the same yellow dense foam rubber...