Seems like a pretty huge block you have there.
Are you planning to bolt it to the spring pack or just a block underneath? For my Blazer, there is a "1-Inch zero rate" spring which acts like another leaf in the spring pack but lifts it another inch (and bolts to the existing spring pack.) It also has a centering pin. These are 2.5" wide though and I don't know how wide our trailer leafs are.