Fascinating bit (which I didn't know) about Tom Costello. When Roncalli was elected, Dr. Elizabeth Gerstner regarded him as a usurper who had obtained his papacy through illegitimate means, was aware of Roncalli's visit to the Masonic Lodge the night before his election (or before the conclave started - I forget which), so while not specifically sedevacantist (she did regard Roncalli as having obtained it, nevertheless) there is something kind of "proto-sedevacantist" about her observations back then. It all just goes to show that Catholics did not ALL sleep through the 1960's loss of faith.
However, the Feeney question most certainly was raging at the time, held only most barely in check by the occasional (but regrettably brief) official theological responses from the Church's approved theologians and disciplinary actions. I do point out that Fr. Feeney did write in Bread of Life (page 42), "The most visible ruler in the world, our Holy Father, in his white robe and white zucchetto, may as well take off his triple tiara and get down from his golden throne, and leave Christianity to the kind of committee arrangements to which it is committed in the present-day America, if we keep on preaching "Baptism of Desire." One gets the clear indication there that had Pope Pius XII been more clear, explicit, and unmistakable about his support, approval, and most of all his total and infallibly-intended support for the contents and findings expressed in the Marchetti-Selvaggiani letter, Fr. Feeney would have declared "Sede Vacante!"
The failure of Feeney and Gerstner (and apparently everyone else) to declare Sede Vacante during the reign of John XXIII seems more like a "failure of imagination" rather than any clear and substantive claims to the papacy on the part of Roncalli - it just never occurred to anyone that someone seemingly elected pope and widely thought of as such would in fact - per the more detailed technicalities of Catholic doctrine - therefore not be a real and actual Roman Catholic Pope. Had the notion simply been breathed back then, I think many would have at least wondered, "there might be something to this." Perhaps the question might never have arisen had Roncalli been followed by a real Catholic. It was the far more odious and ridiculous career of Montini as "Paul VI" which compelled Catholics to begin digging into their theological manuals and discovering the concept of Sede Vacante as expressed therein.