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A rigid frame helps a lot for ride quality unfortunately. A soft frame is basically another spring to flex when you hit a bump, except there’s no damper to control this motion so it can just oscillate, which produces noise and vibrations in the cabin. The stiffer the frame, the more of the bump energy goes into the damper instead (simplified statement but roughly right). That improves body control, tire control (traction), noise, and vibrations.

I don’t know how much you can stiffen an old Chevy frame but I’d start there, and use the smallest diameter wheel possible to get a lot of tire sidewall (acts as an extra soft spring preventing forces from spiking in the suspension). Further improvements can come from lighter suspension/steering components and you can always bandaid with Dynamat or other heavy insulation when you’ve run out of low hanging fruit.

Body on frame car is very old (80’s).

Since then cars are unibody which means the frame part is integrated and spread across the body and floor parts.

Manufacturers have moved on to subframes. A strong U or boxshaped tube rail to fasten powertrain mounts, steering, suspension arm and all moving parts onto. They then bolt the subframe to the unibody with dampers and bushings to reduce NVH into the cabin.

For soundproofing, most windshields now have 2 panes with glue sandwiched between the 2, the pillars are filled with foam, door panels and the floor have strategically placed thick sheets of sound dampening,

As for smoothness, lots of cars have different suspension to do the job. The standard is the multi-link suspension( many control arm with different range of rotation and angles that together keep the wheel flat on the surface if you accelerate, Brake, corner) having as many arms as possible helps the ride stay smooth and quiet.

There’s plenty you can do depending on what you have.

Swapping in new replacement coilovers, changing out the leaf springs for a 3 link… two trailing arms and a panhard bar.

Make it ride and handle a lot better.

Pulling the interior (seats, carpet, door panels) and trunk liner out and installing Dynamat or Boom mat then putting the interior back together. Spraying the bottom with a Line-X type bed liner. That’ll help with noise and helps keep the inside a little cooler.