In many cases the hole in the back of the crank is large enough. Some cranks will have a pilot bushing already installed, sitting in front of the torque converter nose. What if you do not want to cut the input shaft? If it is accessible, and the hole isn't big enough, it may be preferable to drill the back of the crank rather than shorten the transmission input shaft. As it is not a precision operation one could use a hand held drill.

R.