Options to stiffen chassis 69 Firebird?

[Tinker]

My Firebird has subframe connectors I welded to factory subframe. I included two more cross braces that also serve as driveshaft loops and two more diagonal struts. The ridetech rear cradle for the four link stiffens out back. Don't underestimate the additional stiffness from solid body mounts. I can have the front on two jackstands, Jack up in front of one rear wheel, the car comes up very level with virtually no twist. It's not perfect, but it's a world of difference sitting behind the wheel. My next step may be some sort of downtubes from firewall to frame.