Mike C
Honestly, I would be tempted to do the block and run it. I've never had trouble with blocks. Now I'm talking out of both sides of my mouth because I'm running an ORD shackle flip for 4" lift. I think the shackle flip will give you rear spring options galore, and you could combine with a zero rate for leveling if needed. I am running the zero rate to move axle back 1" and I get an additional 1" lift for leveling... I really like the stance.
And on leveling. It seems like every lift I've gotten my trucks squats in the rear. Yours seems to be different. I'm wondering about your spring perches. Are they stock?
And I've never had issues with a rear shaft on a 4" lift. Maybe someone got the wrong shaft before you got the rig.
I think of you get a flip, you might get more spring options for cheaper rear springs. Maybe even free from someone.
Good luck
j