The brakes were a constant source of trouble. They did a poor job of stopping the car. The brakes always chattered.
The lining were replaced at least 5 times.
The car never stopped in a straight line. I had changed the wheel cylinders at least three times and that helped for a little while. Brakes would bind up on one side (usually front) and you'd have to drive the car to heat them up so they'd release. You needed to beat the drums off the car with a drum puller, to check brakes or do any type of work.
The car had a torsion-bar suspension and did not have a very comfortable ride. The frontend was always off and the steering was not always comfortable, as the power steering was some what over assisted, and it gave you a poor feel for the road.
The engine performance left a lot to be desired. It was a 318 V8, but honestly it was nothing to get excited about. In fact I found it to be rather sluggish even after it was tuned up.
Maybe it's still under warranty??
Your proportioning valve was most likely sticking, or you had a bad brake hose, causing the brake problem the whole time.
New bushings and shocks would have fixed the handling, and then you didn't mention the tire type, age, etc.
The 318 Poly wasn't supposed to be the race engine--the 350ci B block with dual quads was still a high 15 second car!
Proportioning valve? On a '58 Plymouth?
Got to remember that the car was 48 at the time of writing. It could have been involved in an accident or two in its life.