Re: 60's schwinn cruiser
The shift kit will allow you to use the motor with your multi-speed sprockets, instead of using a completely different drive sprocket. Not necessary by any means, but allows you to have torque and acceleration when you need it, or top speed.
SBP sells them.
If I were to do my cruiser build over again, I would definitely choose a bike with gears and a shift kit. In fact, if my Cranny frame collapses into a smoldering heap, I will do just that.