60 yrs is a hiccup in history. Asians have long memories. The Japanese are an imperial nation. Always were. Always will be. The question is will they alter their constitution if confronted with an existential threat. The answer IMHO is to hell with the constitution.
I am hoping that they are just blustering, but that it works. I don't think it would be good for the US if Japan had the bomb, for a simple reason. Our influence in that region is based on the fact that we provide the nuclear umbrella for all of the free nations in that region. If they each had their own a-bomb, they would not need us, and our influence would recede.
What I'm hoping is that the threat from Japan motivates China to change its tact and get rid of the fool in N Korea. They are better off dealing with the US as a nuclear power in their region than they are dealing with a dozen little nuclear powers.