Any guesses on one of the most popular center sections out there?
Yup, the Ford nine-inch! They're popular for a few reasons:
1. They're easy to find. Manufacturers started making them around 1957 and ran them all the way up through the 80s.
2. They're tough.
3. They have a drop out center section. That makes swapping the gear ratios around fairly easy.
The aftermarket world has really embraced these and you can build a new one straight from the catelog or buy a pre-built one from a number of manufacturers. Either way, make sure and double check your measurements as there's also an 8" option along with a 9 and 3/8" (known as a 9 1/2") option.