Suspension advice

Dump the 44 year old 0.82” slant six bars they are too mushy, and the pro stock springs out back with their different rates and arch. Replace with ride height of choice six leaf springs from ESPO, and anything between 0.940” and one inch torsion bars. Rubber lower control arm and upper control arm with off-set bushings because you are using low profile tires, two piece strut rod bushings, and 1 ¼” front sway bar preferable not an Addco with their junk mounting hardware. Four Monroe gasmatic shocks will do for now.

Make sure all steering components are tight and within wear spec including the steering gear. Having all parts tight will make a huge difference in handling.

Also install sub-frame connectors, and perhaps an aftermarket lower radiator support stiffener, and torque boxes to tie the front and rear of the car together.

Keep factory ride height set per FSM procedure anything different will impact handling & ride, dial in as much plus castor possible, offset bushings help with this setting. If more castor is needed than what off set bushings can provide, look to installing tubular upper control arms, but get a set that is gusseted at the ball joint, these aftermarket parts generally can give up to 6+ degrees of castor to correct for alignment problems.

Where you are using low profile tires, keep in mind that the suspension designers of the day took into consideration the ability of a tall tire’s side wall to provide some of the cushioning to the overall design. Modern cars take out some of the ride harshness transmitted from inflexible short side walls by using huge rubber bushings, bushings that need constant replacement.

Your old suspension is not unlike the one that came with my Dart, the car was a sloppy unsafe mess that felt as if there were a big hinge connecting the rear and the front of two different cars.

I installed the suspension listed above under my /6 ‘67 ragtop except the lower radiator reinforcement with stock tire size. The Dart now handles like a modern vehicle, flat, predictable, neutral, and no harsh ride, in fact with 0.940” bars, it is softer than my ’07 300C AWD.