There were a lot of Catholics on the staff of the Birch Society when it was located in Massachusetts, a heavily Catholic state, as well as the base of operations of Father Feeney, a forerunner of today's Catholic traditionalists. (One source claims about one-half of the JBS staff were Catholics.) John McManus is reportedly a traditionalist Catholic, and could have been a factor in converting Welch, if he did in fact convert. McManus was the publicity director of the organization, and did for the JBS what Robert Welch did for the Welch Candy Company. As president, he was presumably Welch's chosen successor after Larry McDonald's death. It would be fair to assume that, of the senior staff of the JBS, McManus was closest to Welch.
Given the humanist, rationalist, and determinist (remember the influence of Spengler) strains in Welch's writings like The Blue Book, it seems that a conversion to Catholicism, with its doctrines of original sin, a personal, all powerful, and omniscient God, and free will, would be out of character. However, men's minds can and do change. After twenty plus years of troubles and the rise and decay of his organization, Welch may have become more receptive to the message of traditional Catholicism, especially as he faced his own mortality.
It would be interesting to read McManus's memoirs on the matter, if he ever chooses to write them, or perhaps any correspondence from Robert Welch or his wife discussing conversion to traditional Catholicism.
In a May 1961 letter to Neil McCarthy, Welch wrote that approximately one-half of JBS membership was Catholic, one-half of JBS field staff was Catholic and 2/3 of JBS HQ staff (in Belmont) was Catholic.
McManus was not the heir apparent. Originally, Tom Hill was thought to be Welch's likely successor. (Incidentally, Hill died last October).
Several prominent Birchers have discussed the "conversion" claims. In the final weeks of his life, Welch was reportedly incoherent and, thus, incapable of converting but McManus claims he did convert---which is yet another reason for the rancorous internal disputes within the Society.
Other disputes have arisen concerning religious practices at JBS Youth Camps and the question of whether or not the JBS should welcome non-believers. The most serious controversies, however, concern the influence of Mormon officials within the JBS.