I would try the Moog Cargo Coil CC507s. They are a little taller than SS springs on the back of my SS, but shorter than 1A2 wagon springs. Of course, this is with SS weight, not wagon weight, on them. The CC507s should compress further than the wagon springs in back. I'll betcha it ends up dang near perfect!
I got my set from carparts.com for approx $75 shipped to my door.
And yes, you can easily bend the load-leveling bracket to accomodate the shorter springs.