In Community, Covenant And Commitment: Selected Letters And Communications, edited by Rabbi Nathanial Helfgot (Ktav, 2005), there are at least two statements by Rabbi J.B. Soleveitchik on humane slaughter:
1.5. Statement of Rabbi Joseph B. Soloveitchik to the National Advisory Committee to the Secretary of Agriculture, July 21, 1961 (p. 69)
1.6. Statement of Rabbi Joseph B. Soloveitchik to the National Advisory Committee to the Secretary of Agriculture, October 6, 1964 (p. 71)
I'd like to see these statements and perhaps post them. (Because these are government documents, there is no copyright on them.)
If you have or have access to this book and a scanner, a scanned pdf of the two statements would be very helpful.
Any other documents on shechita or humane slaughter in the book would be helpful, as well.
Thank you!