That is bach's cello prelude from suite no1? I used to play that on the piano, and I always stretched.
As you get more advanced, you will be playing pieces that require this kind of stretching, and you will learn how to quickly adjust your hand in order to allow your fingers to go where they need to go. I am not sure what finger substitution is in this context, but I imagine you mean to incorporate your right hand to hit those higher pitched notes?
I would recommend spreading your hand. With practice and starting slowly, it will come naturally just like everything else.
Or better yet, do both!
EDIT: I should note that after 10 years of playing (but only one year of lessons, so take my advice with a grain of salt) my left hand pinky tip to thumb tip is substantially longer than my right hand. Just like with guitarists, your hands over time will stretch out and be able to play those kind of passages, but you must practice obviously.