In comparison to the way my '70 Monaco DH43 is constructed in comparison to my '77 Camaro, the DH43 is light years ahead of how the extensions of the front (what I'm going to term "sub-frame", correct of not, for comparison purposes) sub-frame actually go farther back on the floorpan, much closer to the start of the rear suspension "frame", than the Camaro does. A much stronger situation, it appears to me. One of the first things you do with the GM F-bodies is to attach the two "frames" together. They are in a more linear relationship than on the Chrysler C-body floorpan, so it's easier. The rear of the GM F-body sub-frame connectors is the rear spring eye bracket attachment.
Your front body supports might help with the movement of the fenders on a rough road, BUT I'm not sure that will have any benefit in auto-cross-style handling.
On the GM F-bodies, one of the other things that is usually changed is the sub-frame mounting rubber insulators. Usually replaced with hard urethane plastic. On the Aspen/Volare chassis police cars, and the R-cars, there were steel pucks to mount the front sub-frame to the body, replacing the large rubber insulators which came on these cars from the factory.
To me, much of what you've already done or plan to do with the additional bracing and such can tend to be more "show" than "go". Rather than those fancy wheel hubs, put some additional bracing in the rocker panels, front to rear. In addition to any sub-frame connectors you might jig-up.
On the GM F-cars, there were some later braces which connected from the rear mount of the lower control arm to the sub-frame itself. When I put them on my '77 Camaro, I could tell a difference in how solid the front of the car felt. On other cars, they have some braces which go from the front of the frame to the center of the "crossmember", one on each side, splayed as necessary.
ONE thing about station wagons, compared to similar sedans, is that while the C-body sedans were usually 55/45 f/r weight distribution, the additional weight of the wagon body, over the rear axle area, should put it closer to 50/50, I suspect. That can have some dynamics on the ultimate "tuning" of things.
For the '66 Chrysler wagons, they spec'd the front tire pressure at something like 22psi rather than the normal 24psi base pressure. On the rear of the wagons, it was more "load-carrying" with almost 30psi. What that means is that the front tires would slide well before the rear tires would, ensuring a strong understeering vehicle. On the sedans, starting with the base 24psi, adding 4psi for "freeway driving", that puts things at 28psi. But with 28psi in all four tires, with more weight on the front tires, they will still slip sooner than the rear tires would, so I increased the front tire pressure 2psi to compensate for the additional weight on the front tires (30/28, f/r). This increased steering response a good bit, as now each axle's tires were supporting an equal amount of weight with their inflation pressure, relatively-speaking. Rear tires wore evenly across the tread, rather than more in the center. Same with the front tires. It just worked better and didn't cost one cent to make happen!\\
Now, why would anybody deliberately want their cars to understeer in a turn? "Safety" is a significant issue! When the front end starts to slide, the driver will usually slow down. That saves lives and sheet metal, usually.
NOW, remember too that any car platform is designed to have "flex" in it. Making certain areas stiffer will mean that "removed flex" will end up somewhere else in the platform it was not designed to be in. This can cause undesired "stress cracks", by observation. Typically where two panels meet and are welded/soldered together. On a Chrysler UniBody platform, these joints can be hidden by a thick coat of "seam sealer" that's painted over as the body is built.
When you remove compliance from the front suspension, with urethane bushings and body mounts, you transfer all of that "flex" into the floorpan itself. You might experience sharper handling, while feeling and hearing more of the road at the same time. One of the main load-bearing areas of the floorpan area are the rocker panels, so I'd consider a stiffening "pipe" in that area, front to rear.
On the rear suspension, using a urethane front eye bushing for the rear leaf springs can help a good bit for more accurate rear axle positioning. OR at least a harder rubber bushing. On the GM F-cars, there are urethane insulators for the rear leaf spring mounts to the rear axle, to replace the factory rubber items. On the Gen 2 Z/28s, there was also a harder rear shackle bushing. I put those on my car and it made the rear end feel more solid with less side-sway. On the stock bushings, I could squeeze them together with my finders, closing the center hole, but the Z/28-spec items were stiffer (higher durometer rating) that I could only get the hold half-closed, for example.
Dig up the Chrysler MasterTech course on front suspension alignment from either
www.mymopar.com or
www.onlineimperialclub. It'll graphically display how the outer wheel goes into negative camber in a turn, as the inner wheel goes into positive camber, leaving the front wheels more vertical to the road surface as the car body leans in the corner. With decreased body lean and much wider tire tread/rim width, you can easily end up with just the outside tread ribs doing all of the work, rather than the full tread width. Wouldn't matter nearly as much with a typical GM car, but due to the Chrysler geometry, it can.
Going to a Z-rated tire is a good move. Generally stiffer sidewalls and tread. Plus a much better tread compound. Look for "auto-cross-specific" tires as their rubber has to "start working" in by the first turn of the course. As in the old Yokohama A008s, the outside tread rib is pretty solid, too.
Although You've already done a good bit, I'd highly suggest you seek out the region's SCCA
"Solo I" Autocross competitions/track days. Usually at a school parking lot or older airport runway. Watch the cars as they go through the course. How some cars slide as others seem to be on "invisible rails". Generally, the courses are designed from shorter-wheelbase cars, as you'll quickly see. Anything longer than about 110" can be difficult to get through the course, by observation.
You might also want to put some extra thickness in the rear crossmember which is the upper rear shock mounts. PLUS make sure the upper shock bolts will remain tight, so they won't loosen and wallow-out those mounting bolt holes!
AND . . . don't forget about brake upgrades, too!!!
Just some thoughts,
CBODY67