In the case of the Spyders neither the bodies or the donors ever seem to be symmetrical. Now I've put my bonnet on I can see that my front cross member is actually sitting at a bit of a jaunty angle to the direction of travel of the car, which has also put the radiator at the same angle because I've bolted it to the orignal radiator mounts on the cross member. This looks to be from a front-end shunt that the donor had at some stage in its life
Luckily it doesn't seem to have affected the chassis any further back.
The trick is to fit the body with the wheels centred in the arches and with even gaps above them, and not to worry too much about side to side symmetry. How often are you going to be able to see both sides of the car at the same time?