Here we go...
http://forums.hybridz.org/index.php/topic/91330-240z-chassis-prep-reinforcements/page__p__864737__hl__%2Bchassis+%2Breinforcement
For a street driven S30, here's about as much as you can do to stiffen the chassis:
1. Seam Weld (every sheet metal seam overlap.)
2. Bad Dog Subframe Connectors
3. Weld-In Roll Bar.
4. Welded In Rear STB.
5. Triangulated Front STB.
6. Radiator Core Support Reinforcement.
7. Transverse Link Bracing Reinforcement.
8. RT Diff Mount.
9. Tubular Seat Mounts.
10. Box Upper Frame Horns to Firewall/Cowl.
11. Reinforce Pedal Box/Brake MC Mount.
12. Reinforce Front ARB Mount.
13. Reinforce Front Strut Tower to Frame Rail Junction.