If you put blocks between the frame and front spring hanger all it will do is lift you truck more and throw your pinion angle out of whack.

First how long of shackles were you running, I would try a six inch shackle, if that doesn't work you could do something similar to what I did.  http://board.marlincrawler.com/index.php?topic=17097.120  Here is the link to my build, i don't have the best pic to show you how I did it but scroll through pages 4 and 5 and look at how I mounted my shackle hangers under the frame with a six inch shackle.

This will solve your problem, however you may have have to lower the front hanger as well to get your pinion angle where you want.  I will take some measurements when I get home so you know where mine are set up at.
                                                                 
I have already fully cycled the front suspension and put the spring way past neg. arc, and still had plenty of upward travel out of my shackle as well as droop. 

I may post some better pics but I don't know where my camera is