Commentary on Mopar XHD springs.
FWIW, my 68 Barracuda had three options one of which was heavy duty suspension (\6 car). It had identical springs on each side with 6 leafs each. They were tapped out and the rear was sagging (needed longer shakles to keep the car level). I purchased a set of Mancini XHD springs to replace them. They are 5 leafs per side, but if you measure the thickness of the spring pack at the axle the 5 leaf set was about 0.030" thicker than the 6 leaf set. Each spring leaf was a little thicker than the ones in the 6 leaf set.
Anyway, when installed the rear sat higher than I liked (yes I installed correct shackles), but within 500 miles the rear settled and it has been the same now for about 20k miles. Here is a picture of the rear of my car.
FWIW, my 68 Barracuda had three options one of which was heavy duty suspension (\6 car). It had identical springs on each side with 6 leafs each. They were tapped out and the rear was sagging (needed longer shakles to keep the car level). I purchased a set of Mancini XHD springs to replace them. They are 5 leafs per side, but if you measure the thickness of the spring pack at the axle the 5 leaf set was about 0.030" thicker than the 6 leaf set. Each spring leaf was a little thicker than the ones in the 6 leaf set.
Anyway, when installed the rear sat higher than I liked (yes I installed correct shackles), but within 500 miles the rear settled and it has been the same now for about 20k miles. Here is a picture of the rear of my car.