OK, now, back to basics........ I'm checking the parts book for '70/'71.... For '70, the 440 and 426 equipped B-J's (Barracuda and Challenger), there are two different part numbers for the springs, 3420176 and 3420177. By this, there are two different springs and I believe them to be the 6 and 7 leaf versions.. Now, for '71, the book calls for the left and right (quantity 2) to be part number 3549059. Those I believe to be the 6 leaf versions on both sides........ Soooooo, although the Mopar Performance version of the XHD springs may be wrong for the '70 model year, it looks to be fairly more accurate for '71......
Now, the '71 B-body is a different story. There's still two different part numbers reflective of the 6 and 7 leaf springs......
W - Charger/Coronet is part numbers 3004767-8
Troy