i'm making a semi-custom suspension set-up using 52" springs front and rear of my rig. 4" lift in the rear/6" for the front. i was wondering if you can tell me what the distance is between the actual solid mounts were on a stock chevy running these springs (i mean the actual distance from spring hanger to spring hanger...not the shackle but where the shackle mounts).
does this distance change when you do the shackle flip?

Hanger to hanger does not change with shackle flip, you just flip the shackle. Sorry, I have no way to measure the hangers hole to hole. BTW, I just moved mine to get the geometry (shackle angle among other things) that I wanted. I wouldn't worry too much about the factory location. Just do what I did, clamp the hanger one with a couple of C clamps, ease the trailer onto the springs to see how it sets, move as necessary, repeat...