Two separate issues here... First leaf springs, then shackle material.
Leaf springs have a spring rating in pounds measured by the amount of deflection by X number of pounds of force (simply stated). The same size spring could have more or less spring rating based on the metal, the temper, etc. So, a spring that simply fits a bracket may or may not be the best overall spring to use -- yes, you would gain a lot of lift using a higher rated spring -- but at a cost. The rear of your truck would be almost like a pogo stick, versus having a correct spring rate suspension that would allow the springs to flex while going over obstacles.
I've seen a lot of guys (trying) to wheel something that has incorrect spring rates. They're all jacked up, but can't get anywhere because the truck just hops versus plants a tire and goes. Same goes when too low a spring rate is used. Then, there is a resultant lack of control as the spring just deflects over every obstacle, turning the truck into a baby carriage, bouncy-bouncy, and never plant and go.
This is also important up front, and it is difficult to actually find another coil spring that is rated as high as the ones that come stock on a TTB front end under an explorer. Due to the short lever arms, the front springs are rated much higher than a similar sized vehicle with a straight axle, i.e., a Jeep Grand Cherokee. The Explorer would run close to a 500 # rated front spring, while the Grand would run closer to 300 #.
Shackle material...
It does not have to be made out of spring steel. Any decent grade of mild steel will work fine. Besides, it is virtually impossible to machine spring steel, spring steel is brittle when used in an unsupported manner, and it is simply not the correct material for the job.
I used 3/8 common bar stock for my own extended shackles and they've taken every sort of abuse I can hand out to them with no problems. Most guys use 1/4". To use 3/8", you also have to find a source for longer metric bolts, which can prove an interesting search. They are not on the shelf at the local hardware store...