Shackle box grinding?

I recently bought and installed some BOR 6" springs. They are longer than stock, so on larger bumps in which I have a lot of compression teh shackle hits the back of the box it is in. KLUNK. I am currently running the stock shackle until they break in and then my lift shackle will go in. My question is, how much of that shackle box can I grind away without affecting its structural integrity? I have been told NOT to run a boomerang to help allow the spring to have a limit to how far it can compress. So, just curious if any of you ahve thoughts on this. Thanks
josh

Why not run some sort of longer, temporary bumpstop as opposed to the shorter shackle.