I finally got around to testing this. Yves memory was spot on. I tried fitting a 69 Berlina tank in a 73 GTV. The Berlina's neck is a bit higher and sticks out enought that the GTV's fuel door won't close with the gas cap on. Makes sense when you think about it, the Berlina's rear quarter is basically a verticle slab whereas the GTV slopes back slightly. The footprint of the tank itself is the same so a good radiator shop could probably swap necks but it might be tricky getting the angle just right. Might see if they could tack it for a trial fit before welding it up. Welding on a gas tank is NOT a home project.
Bill Sinclair