Sounds like you have screwed the old fitting in the new tank and it leaks, right?
That says to me that the difference in the pipe thread sizes and pitch is very small.
I would take the fitting out, get a new one as the threads may be damaged.
And then use a NPT tap to re cut the threads in the bung to match the new fitting of the original size.
And why Chevy Duty does not do this before they send the tanks out I don't know?
Good luck
