I get your frustration. How difficult can it be to get this correct? Crappy reproduction parts are the bane of the restoration business.
I guess I might try repairing up your tank you had to bend the neck on. However it's a big PITA after it has had fuel in it. I have done it before by draining the fuel and filling the tank with water before working on it. I don't recommend it as bad things can happen.
Like others suggested you might find a good tank from someone that has changed theirs to the box.
Another option other have mentioned. Here is a Eastwood video of how to clean and seal a tank.
https://www.youtube.com/watch?v=Covk_AxA3Ig
Quanta (from Canada) has a great reputation on correctly made tanks.
https://www.gastanks.com/Pickup-C_K-...products/1958/
You might give them a try if you haven't already.
Rick