Yup, I make my own out of 3x2" square tube .100 wall. I slot open the front part of the rear sub-frame in front of the front leaf spring perches. The 3x2 tube slides right in. The 3x2 tubes protrudes through the floor about 1" in height in the area where the rear passengers feet would go. I just cut the carpet's insulation over top of the connector and throw in my floor mats after and it's not noticeable. The part under the front passenger/driver feet area is under the floor so no modding needed there. Just weld it up to the torsion bar crossmember and in the back and you're done. Makes a huge difference in chassis rigidity. Before if I had one corner jacked up in the air, my doors would be very hard to open, now they open/close smooth even with one corner high up on a jack stand.

Here's a pic of my brother doing the same thing on his 66 charger while he was replacing rusted floor pans.