I have nothing to reference but as a Chevy fan and Automotive trainer I remember this story going back to about 1967. Mouse motor name came from the Mighty Mouse cartoon. Because other manufacturers were already in the 300 pluse Cu In at the time of Chevys development of what we call the small block now, it was not called that when it first came out. It was just Chevys entry into the V 8 field. At 265 Cu Ins it preformed past all expectations. They could have easily called it Superman or Batman because of its super performance. Engineers and performance people called it Mighty Mouse. (I do not know who coined the name) The name really didn’t take off until the big blocks came out from Chevy and sometime after that Mighty was dropped and just called the mouse motor with the Big Block being known as a Rat motor because its bigger than a mouse. The popularity of these knick names really didn’t catch on until about the mid sixties.