Check out XV Motorsports. They are pretty deep into frame stiffening. Pretty pricey stuff but it's a good learn. See their 4 post videos, very interesting.

The shock tower supports and extending the rockers are great ideas. Connecting the subframes together is a great drag racing move because it transfers some suspension load to the floor and most imporatantly the front subframe. As for torsional ridigity, connectors by itself is overrated.