What's this part called?

The factory ones are hollow and about the same weight as a piece of 1/2" EMT of the same length. Maybe a scooch heavier. Ex-sparky. The engineers had to convince the bean counters that they were worth the money.
I think they were more for the uni-body flex thing rather than for crash requirements. They started on B body in 72 and A body in 74. E body never had them.