Chassis Stiffening
If your intent is to build a competition autocross car, I would decide what organization you're going to race with and what class you want to be in before you do anything in the way of chassis stiffening. Some organizations (SCCA) are pretty strict about what they allow in their classes, and you don't want to end up in an "unlimited" type class if you want to be competitive. So, if that's your intent, get a rulebook before you get to work.
Now, if your goal is just to have fun, do whatever you like. :D There are a few places now to do autocrosses with fairly open classes, like at GoodGuy's or USCA events.
I really don't think there would be a huge difference in chassis rigidity between the fastback Barracuda and the Duster. The wheelbase is the same, and that's probably one of the biggest considerations for autocross. Keep in mind that while the fastback doesn't have cross bracing, it does have other bracing to make up for it (just like the fold-down seat Dusters have additional bracing to make up for the lack of the X brace). Once you've added subframe connectors and torque boxes its not going to be a make or break situation, I doubt you'd see much of a time difference on the track. I added a fold down seat to my Duster for utility, and after doing so I'm pretty certain the fold down cars weren't any less rigid than cars with the X-brace in the back. I have a whole thread on my fold down seat conversion, it details all of the factory bracing pretty well. http://www.forabodiesonly.com/mopar/showthread.php?t=258695
As for the rest of my car, I fully seam welded the K frame, reinforced the LCA pivot and steering box mounts, added a skid plate, boxed the LCA's, reinforced the torsion bar anchors, added subframe connectors, torque boxes, a tubular radiator support brace, and "J" bars that tie the firewall to the frame rails and support the shock mounts. It make a HUGE difference, the car seems very solid now.
If your intent is to build a competition autocross car, I would decide what organization you're going to race with and what class you want to be in before you do anything in the way of chassis stiffening. Some organizations (SCCA) are pretty strict about what they allow in their classes, and you don't want to end up in an "unlimited" type class if you want to be competitive. So, if that's your intent, get a rulebook before you get to work.
Now, if your goal is just to have fun, do whatever you like. :D There are a few places now to do autocrosses with fairly open classes, like at GoodGuy's or USCA events.
I really don't think there would be a huge difference in chassis rigidity between the fastback Barracuda and the Duster. The wheelbase is the same, and that's probably one of the biggest considerations for autocross. Keep in mind that while the fastback doesn't have cross bracing, it does have other bracing to make up for it (just like the fold-down seat Dusters have additional bracing to make up for the lack of the X brace). Once you've added subframe connectors and torque boxes its not going to be a make or break situation, I doubt you'd see much of a time difference on the track. I added a fold down seat to my Duster for utility, and after doing so I'm pretty certain the fold down cars weren't any less rigid than cars with the X-brace in the back. I have a whole thread on my fold down seat conversion, it details all of the factory bracing pretty well. http://www.forabodiesonly.com/mopar/showthread.php?t=258695
As for the rest of my car, I fully seam welded the K frame, reinforced the LCA pivot and steering box mounts, added a skid plate, boxed the LCA's, reinforced the torsion bar anchors, added subframe connectors, torque boxes, a tubular radiator support brace, and "J" bars that tie the firewall to the frame rails and support the shock mounts. It make a HUGE difference, the car seems very solid now.