One thing I've noticed too is your rear tires appear to be much taller than the front. A lot of the pics people are posting the tires look to be similar height front and back. If I get home at lunch today I'll check my shackles on my 65 coronet.
grubby65 my Coronet stock shackles are 5" long and the bolt holes are 3.5" apart. Hope that helps out.
May shackles are also 3 1/2" from bolt to bolt. So is that stock length?