Which flex plate? 153 or 168 teeth
 

Which on do I use?Internal balance SBC. Are the starters different for each one?

The choice is yours. OE starters are different in how they mount, but the mini aftermarket starters are good for either flexplate. I've heard you get more torque to the engine with a 153 tooth, but can't verify that.

Personally, I've run the bigger on all my stuff with no issues.