In this episode of PowerNation Extra, Detroit Muscle host, Tommy, goes into detail on why the Ford 9" is such a popular center section.
There's 3 main reasons:
1. They're relatively easy to find since they started making them back in 1957 and used them up until the 80's.
2. They come built tough from the factory.
3. They have a drop out center section which makes switching the gear ratios around easier.