I believe that there may be evidence to support that Gregory XVII was the elected pope in 1958, but I haven't found concrete evidence to support it. I am looking into it to the best of my ability, but I can't say for sure. Whatever I do find, however, I will be sure to share if I find anything concrete to support it.
Right now, I'm a sede impedetist, meaning, I believe the throne of Peter has been usurped by someone that is a pretender in the place of the person that should rightfully be sitting there, whoever it is, but that is not a sede vacantist, who believes that there is no pope.
I hope that clarifies my position.