Start buy measuring from center point of the front crossmember to the the outer edge of the rear axle. You need to determine if the rear is centered to the frame / crossmember. Once you have accurately measured the rear axle move to each side and measure up to some solid point on the cross member with a identical point on both sides. If these two come out right, the only thing left is a bent control arm or adjustment point. I never have worked on a corvette, so i couldn't tell you where that might be.
We used to align dual axle road tractors with dixie cups, heavy string, jack stands, and a big tape measure. The key is to find a fixed point and start from it. Movable joints can be bent or out of place and not really show up.
Joe
