Just to confirm, you bought a Corrado tank expecting it to fit into a Golf2 and the only bit that doesn't fit is the filler, where it should mate with the body?
Let me give you a hand here: The Corrado is mostly based on Golf2 underpinnings - pretty much most of the 'hard-points' are the same, but the body is quite a different shape.
And you're surprised that the bit that doesn't fit is the bit that mates up with the body?
If it's any consolation I know people have successfully fitted Golf3 fuel tanks to Golf2s. A Golf3 tank should be a lot easer to find!
If you buy the tank out of a VR6 this would give you a combined in-tank lift pump and pressure pump to suit your 20VT