Be warned these springs are extremely soft and depending on the vintage and condition of the set, you will likely have a noticeable squat to the rear of the vehicle. If I were you, I'd either by two cheap used sets (less than $50 a pair) or spend the extra coin and get a brand new set from either a Ford dealer / local spring manufacturer.
After I got my truck loaded w/ tools, a full size spare plus installing the top for the trips to the trail in the winter, I had about a 2.5" difference from the front to the rear. Without the top, it's still about 1.5 - 2" different. I've bought another set to add in more leaves into my pack, but have been too lazy to get around to installing them as of yet. Otherwise, I have been pleased with there performance on the trail.
Also, while this is a relatively simple swap to do, this is not a direct bolt-in. Re-location of the rear spring hanger will be necessary and you will also have to contend w/ the off-set center pin location of the Ford springs in relation to the GM springs. Otherwise, your rear tire will not be properly located in the wheel opening.
I acualy just put a set in the front of my Hybrid k5/toy truck. I don't know exactly what they were from. I had 2 packs laying around so I used the main leaf from the one with the thicker main leaf and used the thinner tapered leaves from the other pack.
They are 3" wide, almost 57" long positioned with the short side forward. I moved the front hangers 3.25" farther froward and 3.25" up on the side of the frame. I moved the shackle mount about 2" back in the frame and used a 7" eye2eye shackle. Shackle angle is about 45* at static height. Axle is about 2.5" forward from stock and I gained 12* of caster. I am running 10* shims welded to the front axle to restore caster to ware I want it. This gave me an initial 5" of lift over the stock sagey springs, it probubly settled a bit but I haven't checked since. I am using a set of rear rancho shocks intended for a 4" lift on a K5 now in the front; in the stock location. They have about 5" of up travil at static height and bottom out on both up travil and down travil. I use just over 7" of slip in my front drive shaft. With 44" tires I have a 70 degree aproach angle, the frame is cut right infront of the spring.
Over all I am happy with the setup. It gives a tremendous amount of travel, raised my RTI score 250 points. Pushed my wheelbase to about 111". It is also surprisingly stable. When I ran it up the ramp I thought it would be trouble on the highway since it was so flexy, but it realy doesn't seem bad. Same thing with side hills.
I have a buddy who has been using a similer setup on a full sized full bodied ford for 4 years now. He uses an extra main leaf, re drilled the springs and has shorter shackles. But it has worked very well for him.
I still need to get some longer shocks/or reposition them to get more uptravil so my bumpstops hit before they bottom out + some limiting straps to keep from hurting them on down travil. Also need to clearance my drive shaft yoke at the tcase because if I am at full droop it is very close to binding. Axle wrap at full droop might just make it bind.
man that sounds sweet.. sounds like a lot of work, but sounds well worth it.. im thinking about just ordering some new Superlift heavy duty 4" springs for the front cuz they 3" ones just arent cutting it, used to be 3.5" but the heavy ass diesel engine is taking its toll... 160 bucks from 4wheelparts, so im gonna do that for now, and then eventually i want the AK-57's in the back.. think it would be bad ace!!
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