I did some looking over the weekend and can't find my reference for this. However the Performance tuning for the Tiger Guide is incorrect in this regard. Also, the book, "How to rebuild Small Block Ford Engines" documents on page 27 that after Feb 63 that the 260 block also changed to three core plugs and the wider spaced engine mount holes. However, these blocks cannot be bored to 289 ci even though they externally resemble the 289 casting. So you can try boring to 289 but all the evidence I've seen says it won't work.