I never replaced the tank in my '72 K5 with an aftermarket one,I did find a nice used one that was a GM replacement that was only a year old when the truck it was in got wrecked...
I do recall the filler neck being as you said,swan like ,and I had a hard time sourcing some genuine filler neck hose,which cost 15 bucks a foot to install it..
I had an 82 GMC Cabbalaro and when I bought a Spectra gas tank for it,the place I got it from said they would have to remove the filler neck off the new tank and my old one,and solder my original one off the El-Camino tank in order for it to work (which they did for no extra charge),so I could put it in myself...they evidently made one aftermarket tank that was also used on Malibu station wagons fit the El-Camino by swapping fuel filler necks instead of making another part,or perhaps they just had no source for the OEM filler necks on the El-Caminos..
Perhaps this is the same issue on your tank..maybe it also fits some other application if you swap the filler neck..but I would think they would specify that in the listings or on an instruction sheet that came with it ?..