Flywheel for my 351C

Same as the other small block, just make sure to get a 28 oz imbalance and the right tooth count, I think C's have 164 teeth... Check out Jegs for prices, they have a few different flywheels listed, but I'm using a factory flywheel in my car most likely. I'd stick with a steel one.
 
Are you asking about a flywheel or flexplate?
Flywheel: They interchange between SBF, Windsor, and Cleveland. You need to make sure you match bellhousing to the flywheel. The 10.5" clutch setup uses the smaller flywheel and aluminum bell. This is common to most SBF. Some trucks and most Clevelands use the larger 11" clutch. The flywheel and bell are larger. The bell is cast iron.
As long as you use the 28 oz stuff, it all interchanges fine.
The starters are the same if you use the correct bell. (auto and manual will not swap though)
If you are using an aftermarket bell, the larger flywheel will fit, but it is very close and the bell may have to be offset slightly for the starter, which skews clearances even tighter.

Flexplates are even easier...
Swap at will. (28 oz only of course)
Dave