Steering All Over the Place at 55mph+

I just jumped in here;
The best way to describe it was it felt like if I made any turn at at all (as simple a merging into another lane) I was going to lose control like it was going to twist (best words o can think to describe it). Even worse when hitting bumps or uneven pavement I thought that was the end for me and struggled to keep it in my lane.
I just jumped in here to say I had a care exactly like that. I hit a train crossing in town at 40 mph and the car just took turned right on me and into the weeds we dent. What a scarry ride that was. I nursed it home and finding nothing, I dug deeper.
What I found was a rotten Passenger side frame that had torn most of the spotwelds from the apron to the firewall; and the only things that was sorta keeping it together were the fenders and a few spotwelds.
Seeing as this was a 50 dollar car, and it was ~1979, That 74 Duster was on it's way to Duster-Heaven, in short order.
I still have most of her, minus the body, in a shed out back. I might still have the "Powerbulge" hood, and one tail-light.
Just so you know, I also have a Barracuda, but a 68 fastback. It cruises real fine at a 100; and is not at all scary at 120 ........ in a straightline. I haven't tried turning it at 120 so can't say what it might be like. But daymn the wind-noise at 120 is ferocious.
And, yur likely gonna need 1.03 T-bars.