Not sure if this will help, but....
I have a 1970 Buick Electra 225 Convertible. I had to do replace my fuel tank and it was an ordeal. Here is what I did. I located a Spectra GM61A fuel tank which had the same dimensions as my old tank except that the filler tube was different in angle for sure. I had a rad shop take the old filler tube, with the cap, off the old tank. The new tank's filler tube was taken off and replaced with the old filler tube. Spectra would not sell to me directly but the Rad shop had an account and I paid the Rad shop for the new tank and the brazing/soldering of the old filler tube. the total cost to the Rad shop was about $700 (Canadian funds) and $150 for my mechanic to switch it out.
Yours is a 1968 Electra so I am not sure if this approach would work, but you can start the investigation process. I see from my Buick Catalogs (Illustrations and Parts Books) that the 1968 to 1970 Electra tanks were similar although the tank part numbers for those years were different.
The new tank with the old filler tube fit perfectly. I did this about 8 years ago. My old tank was 25 Gallons and the new tank is also 25 gallons. All dimensions of the new tank (other than the filler tube) were the exact same. corners placement of the sending unit the breathing vents on the corner etc. The sending unit was also transferred and fit into the new tank perfectly.
Good luck!