I just replaced my stock tank with new one with the correct filler neck. A few different vendors sell it but the best price I could find was from these guys:
http://www.truckandcarshop.com/
They had an ebay ad I found:
https://www.ebay.com/itm/1969-1970-1...0AAOSwOdldO2Ol
It installed just like the original tank except the locating tabs on the sides of the tank must not be in the correct location. I needed to modify the two brackets that sandwich between the tank flange and the frame rail. I left the middle hole/tab alone since that is what centers the tank in the frame rails and instead had to open up the two outer holes so it would match the spacing of the tabs on the new tank.
I can draw a picture of what I did if it helps anyone.