 During his tenure as rector, he oversaw an extensive revision of the curriculum, which earned the seminary accreditation with the Middle States Association of Colleges and Schools; the construction of a new theology wing named Vianney Hall; and the establishment the School of Religious Studies and the School of Pastoral Studies.On February 18, 1970, Welsh was appointed auxiliary bishop of Philadelphia and titular bishop of "Inis Cathaig" by Paul VI.He received his consecration on the following April 2 from Cardinal John Krol, with Bishops Gerald Vincent McDevitt and John Joseph Graham serving as co-consecrators, at the Cathedral of SS.