I would definitely center the trackball.
Most trackball games have the trackball in the center.
Lefties will also be much happier with you if they can use their dominant hand.
With regards to the spinner, I personally would put it to the left of the trackball.
Most spinner games had the spinner on the left, or centered.
The notable exception to this is Tempest.
If you connect the trackball and spinner to separate inputs, you could use them separately to play racing games, Blasteroids, etc...
I would set up the controls as shown on the attachment.
The 4 buttons on the right are set that way for Star Trek.
You could get away with only three buttons on the right, if you wanted to map one of the buttons on the left to photon torpedoes.