Converting 68 Valiant Rear to use 15" wheels

The best route would be-find a A Body 8 1/4 rearend or a 73 up 7 1/4 if you are staying with the Slant 6. It sucks having 2 different wheel bolt patterns on your car. The 73 and up A Bodies with drum brakes continued to use the small 4 inch bolt pattern-so don't get one of those.