Why 2 flywheels for chevy?

Okay guys here is the skinny .........
This guy has a 350 crate motor in his truck and decided to remove his old starter and go with one of those Mini/high trq units. Well things aren't going so smooth with the transition. He asked me for my 2 cents worth and I honestly could not help him. I dont have that problem with my SBF's, I just have to know which god darn flywheel goes with what damper and what crank. Any way my questions and thoughts are as follows.

1. Why do small blocks use both 153 and 168 tooth fly wheels?
There has to be a difference in diameter. I could understand if the larger one was for a BB and therefore cause the starter to have to be mounted further out.

2. Do BB's use both also?

3. Is the 153 used to give the starter more leverage over the crank?

4. Is there any other different mount positions other than the straight and staggered holes?

These are just a couple of questions that I have right now. All information appreciated.
:whacked: