52 inch vs 56 inch spring hanger locations

[1969k20]

I'm in the process of turning a 71 K20 into a shortbox. I made an accident ordering the skyjacker lift in that I got the 52" rear springs instead of the 56" ones that came with it originally. I figure my only options are to send them back (paying shipping both ways) or moving the rear spring hangers forward. The problem with option 2 is that my 3/4 ton frame doesn't have the 52" hanger holes predrilled. Would anyone out there with a 1/2 ton 4x4 frame be willing to measure how many inches forward i need to drill the new holes? I would really appreciate it. Also, will grade 8 bolts be sufficient to remount the hangers to the frame? Lastly, any negatives to using the 52" springs vs the 56" ones? Ride Quality or towing capabilities? Its going to be used as a part time work truck and tow a car trailer occasionally. Just wanted to get your guys opinion if i should get the 56's or use the 52's. I attached a pic i found on this site of the measurements I'm looking for. Thank you guys!