Of course I’m aware there’s multiple gauges of sheet metal on these cars, I’ve welded to all of them so yeah, I’m aware.
I’m sure you’re aware that unibodies are a sum of their parts and that seams, joints, contours and stamped reinforcements are more important than just the simple gauge thickness. The “vent area” as you call it, the upper cowl, is very strong in the corners (where the J bars attach) because it is tied into the seam with the firewall, the seam with the foot box/floor pan, and the seam with the a-pillar and inner fender. Stay close to the corners and you’re dealing with one of the strongest parts of the car. That’s not the “vent area”.
Regardless, I’m more than happy with my J-bars and the noticeable difference in chassis stiffness they made when I added them. Running 275’s up front shows you a lot of flex in these chassis that running tires typical of street drivers and drag racers will not.
US Cartool and XV Engineering have done some of the most extensive chassis testing on these cars, and both offer varying styles of inner fender braces. To reinforce the area between the shock tower and firewall/cowl area.
Exactly like go-fish said, those are the US Cartool braces just being sold by Classic.
Same ones I installed on my ‘71 Dart. They’re inside the fender so they’re less visible than J-bars in your engine compartment, but the way they tie the sock tower into the inner fender, firewall and cowl creates a significant structural member similar to areas of the frame or rockers. The aftermarket companies that have tested chassis flex in these cars all offer reinforcement to that particular area. And it makes sense based on how the original chassis was (and wasn’t) loaded by the torsion bars.
