poor handling on highway

[montanass]

so i'm getting tired of chasing my car around going down the highway. It seems to jump from one side of the rut(s) to the other, making it quite a handfull and takes away from the fun of driving.

My setup is:

69 el camino
BBC- 2" lowering springs, stock spindles, 18x8 front rims 255/40R18 (f)- 18x10 295/35R18 Rear

all hotchkis components, sway bars, lower conrtol arms (rear) edelbrock adjustable upper control (rear), etc.

I'm just wondering if this is common with larger wheels and low pro tires, or if i should start looking into this a little deeper.

thanks