hey, i just been checking some stuff on my car, tried out some 10x18's. and it seams there is a difference left to right. wheels fit ok on left hand side with a 21mm spacer but foul the body mounting on the right side

 did vw change the trailing arm on the beetle in it's life? wonder if i have two different ones? they're standard 1303 arms, as far as i know. and have early 944 brakes. the car has had lots of welding in the past so may be twisted? is nearly 10mm difference! where can i measure from to check?