9" disc (use or not?)
 

what are the pros & cons to a 9" rear disc instead of a 12 bolt?

Easier to narrow, Easier to swap gears by doing a center section.

The nine inch is to rear ends what SBC's are to motors......everyone runs them making parts easier to get and more aftermarket parts available for those.

Here is mine: http://www.hotrodsbyglen.net/68ninec.jpg

the only downside is that they're heavier.

AT LEAST YOUR ADDING WEIGHT WHERE YOU NEED IT. ON THE REAR TIRES!

they take a small amount more power to turn