You can call Mac's and ask them. (877) 579-5722
http://www.macsautoparts.com/ford_th...l-1956-57.html
How to measure...
http://www.eatondetroitspring.com/ho...-leaf-springs/
If you have your springs re-done locally the shop may ask if you want a leaf added. It could help keep the rear bumper off the ground, esp. if you have the orig. continental spare tire on the bumper. But then you might need longer U bolts over the axle.
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