Are suspension parts differernt for a V-8 K frame and the slant 6 K frame?

That PST kit is pricey. You can probably do much better by shopping around on ebay and rockauto and piecing together a kit. I did. I re-used parts that were tight, like the upper ball joints and tie rod ends. The biggest slop is usually from the rubber bushings, lower ball joints, and idler arm. But those usually cause just rattles when going over bumps, not what you describe.

It sounds like your car wants to wander. That is usually from a bad toe-in setting, i.e. "toe out". That can result from the front end sagging. Did you set the ride height (per manual) after putting in the heavier V-8? If not, do so first. You can check toe-in with a ruler, front to back across the front tires (see youtube). Before that, just hold a 4 ft level or 2x4 along each front tire and sight at the rear tires. The fronts should be ever so slightly toed in. The idea is that when driving, they are pushed back from play in the steering and become close to perfectly straight, but never toe out (bad).