They go from the lower control arm to the chassis. It's a brace meant to help stiffen the mounting point and reduce flex. I believe it was first introduced on 66' SS 396 cars.
Strangely enough... my 72' which was a factory 350 2bbl/TH350, has them. My 69' factory 300 HP 350 4 speed car does not have them. (I intended to add them)
Here are some pictures...
http://www.66ss396.com/rearend/