Wondering if there might be some hidden rust in the innards of the cowl area? From accumulated leaves or dirt that didn't get flushed out or similar? Which might compromise the strength of the cowl area of the body? Just a thought.
When I was driving our '66 Newport all of the time, I never saw the front sheet metal shake or move. ONE stiff "piece" of the total car.
By contrast, when my '77 Camaro was new, once I got into a wash-board section of road, at an "uncomfortable" speed for the car on it (like 35mph) and as the front suspension did its thing, the front sheet metal also went up and down. I thought I'd broken something! It visibly moved. This was new to me, at that time. The people I asked about it in the GM Dallas Parts Warehouse referred me to another guy I finally got to talk to. "Normal". I thought . . . GM doesn't know hot to build a decent Unibody car, compared to Chrysler.
What was happening was that the insulators on the sub-frame rails were allowing the sub-frame to move a bit. Even just a littke, amplified over 4 feet gets to be very visible when it happens. ONE reason for the bolt-on braces from the fender to the core support area, it seems. When the '79 Firebirds came out, the WS-6 option cars, they included some small braces that went from the lower control arm mount area to a place on the sub-frame rearward of them. The turbo cars had a different one to clear the turbo outlet pipe. All bolt-on deals. I added those to my Camaro and it made the front end stiffer, still. An improvement.
When we got the '72 Newport, I liked the quieter ride and suspension isolation, which came at "a price" in potential sheet metal movement on rough roads, by observation. My '70 Monaco, with the shorter fenders than the Chrysler, doesn't seem to have any deflection issues.
Do NOT take anything for granted! Re-check the torque on ALL body bolts that hold sheet metal items to the front "frame" area of the car. Period. Do this with the car on a dead level paved area. A drive-on alignment rack would be best, I suspect, as it's got to be calibrated "level". Jossle the suspension a few times for everything to reach it's normal level and stress. Then start at the front and move rearward, or vice versa, and tighten EVERY bolt/nut you can get a socket/wrench on. I did this on the '72 Newport, at the recommendation of a Popular Science magazine artlcle on squeaks and rattles, after the first oil change (3000 miles). I came in one weekend from college and did this one Saturday morning. The build torque on many of the fasteners could be "firmed up" with another 1/4 turn, but usually less. When done, I was surprised how more solid everything was! Almost to the level of the '66 Newport I'd done the same thing to over the years (after I got my first Craftsman socket set one Christmas).c
Now, additionally, almost all metal structures have some panel mounting deflection built into them, as a matter of course. IF it doesn't bend a bit, it breaks over time. As in "stress cracks". "Solid" is not really solid, but sometimes more than others.
In a vehicle body structure, body-on-frame or UniBody, there are places that are designed to flex a bit as other places are not designed to flex. First deflection area is the suspension, then the body mounts. Putting urethane in the place of rubber in some suspension locations, pushed the former deflection in that area farther down the line ultimately into areas where flex is not desired OR designed to be. Especially on the vehicles designed before 1990 (or thereabouts), for example. "Designed" not "built", that is.
To me, on a street-driven vehicle, especially one that can see some "dirt road" action, polyurethane bushings might be fine for the upper control arm rubber, but not the lower control arm rubber. On a sub-frame car, polyurethane on the lower body mount insulator, but not the matching top half of the insulator unit. Take some deflection "out", but still allow for decent sound insulation characteristics. Race-only non-street vehicles can probably use polyurethane where ever it can be, by comparison.
Think about it this way, on engines which use a solid metal motor mount on the side of the block, torque reaction can cause the side of the block to crack and break. Whereas the OEM rubber (restrained with a cable, if needed) will not cause any issues in this area. One reason that the "motor plate" or "elephant ear" mounts work best in race cars where deflection is not desired.
Just some thoughts,
CBODY67
