I've done 52s up front on my '76 K10. I don't know what the differences are between '71 and '76 for the swap. But on '73-87s, a LOT of people seem to have to move the shackle mount FORWARD (not rearward, as you just stated). I, however, got away withOUT having to do that. My "52s" actually measure 55" long along the main leaf, so that's part of it. But I also have my B52 brackets mounted in the forward-most setting, and my front spring eyes sit in the forward-most hole of the B52s. My shackle angle is great too.
My front axle sits about 2-3 inches forward as well. No zero-rates or anything either. The only downside is that when the springs compress, my 6" shackles will hit the body mount brackets.