weird steering

[k.s.]

i've got a strange one for us to figure out.
67 coupe 94 5.0 with all late model accessory's (a/c,p/s,alt, serp belt system)
has granada frt spindles w/disc brakes. its problem is,when driving down road,it darts to one side of the road or the other with just the slightest turn of the wheel,if you barely turn the wheel to the right,it darts right,if you turn left,it darts left. and i mean DARTS. have almost crashed it due to this. it does have the original manual gear box. have also notoced the car doesnt turn as sharp as it used to. i fiqure its the ratio of the manual steer box,or is there a difference in the disc spindles?i am building this car for my mom and dont want her to get the suprise i did when i deliver it to her. HELP!