It's the tank - the aftermarket tanks have a straight fill neck and the OEM stuff had one that comes up and then bends around the frame (at least they do on Blazers).
There are a few fixes for this:
1 - find a decent OEM filler neck and connect it to your new tank
2 - adapt a fitting to the tank to get around the frame challenge - some have found aftermarket filler necks and others have used copper ells - see pix.
1st pic is the what the stock filler neck looks like - not like your tank right?
Here is a thread that shows the two tanks and where they are different -
http://67-72chevytrucks.com/vboard/s...d.php?t=696624
Here is a thread where a user found a creative way to have it work with an adapter on the tank -
http://67-72chevytrucks.com/vboard/s...5&postcount=67