Actually I was thinking of the Fords that I feel sure did not have the bars through the firewall. The Chevs all did I think. As did the Bob Jane one here in Oz. though that was ZL1, not a girly Z28!!
If you go here:
'69 Penske/Donohue Camaro - Bruce Canepa web site
Evidently at one point Canepa's company handled the car. If you go through the photos, you'll see the roll cage structure inside the car. On shots with the door open, you'll see roll cage tubes disappear into the side sides of the dashboard. Looking at engine compartment photos, you'll see the tubes come through the firewall and go down to the original light perimeter chassis. In the trunk photos, there are tubes beside the fuel cell. Those come from the back side of the cage and extend to the rear of the car and connect to the original perimeter chassis.
Basically the floor pan is attached to the perimeter chassis. The roll cage extends from one end of the car to the other and is welded to it. The big difference between then and now is that the '69 didn't seem to have much cross bracing in the front and rear like the NASCAR cars do these days.
By the way, roll cages in stock cars began to appear in maybe '65 or '66 (going from memory) by the Holman-Moody organization.
The Camaro front chassis is rubber monted [originally] to the body shell, easier to build and repair as a car but never as stiff as a Mustang in particular. Though the Mopars for the era were pretty good too. I feel the AMCs were built similar to the Camaros?
For mine the Chevs had the best engines, the Fords the best chassis, and the Mopars were a decent all round package with a decent engine, strong body shell though torsion bar front end must have been a pain to make go fast