I feel your frustration, but I wouldn't expect NH to eat the cost of returning a large heavy item that you bought by mistake. They didn't guarantee fitment.
FWIW, I've had to modify, in some way, the last 3 tanks I've put in my work trucks. None of them the tank mfgs fault. I knew the dimensions of the tanks before I bought them.
The previous gen super dutys have a curved reinforcement on the front bottom bed corners that prevents having a flat surface wall to wall up there. Makes mounting wide tanks a challenge. Other times I was just trying to cram 10' of stuff in a 6' bed, lol.