Fuel Tank Fitment

[Droff]

I bought a Dorman in cab fuel tank to replace my old, leaking tank and the new one doesn't quite want to fit right.
It looks like the filler neck needs massaged a little bit to move it closer to the tank and a little to the front as well. Right now, when I slide the neck through the hole in the cab, the tank itself won't sit against the back or bottom of the cab like it should.
Any suggestions on how to 'massage' the filler neck without causing more problems?
Thanks.