In the US the Sterling manual says to avoid drilling into the flanges of the gas tank as this can cause leaks. However, the UK manual says to rivet metal strips onto the flanges to allow the tank to be bolted into the shell.
I recently fitted my tank using the UK method, however, I was very careful to avoid accidently puncturing the tank wall when drilling through the flange.
The tanks lower pressing (left in the picture below) is narrower than the top pressing (right in the picture below), so I can see that it would be easy to make a mistake and drill into the upper part of the tank wall if drilling up from below. I deliberately drilled from the top down to avoid getting too close to the the tank wall.
fueltank.jpg