Hotchkis leaf springs sagg brand-new
Wow, that is LOW.
But, those aren't stock shackles, that's just flat stock with some regular old bolts. Not that it really makes a difference if they're the same length.
I was dealing with Hotchkis just recently myself after one of the heims on my Challenger's Hotchkis UCA's wiped out after less than 7k miles. They were nice enough to send me replacement heims and dust boots for them though, and were right quick about it after I posted about the failure over on another mopar board.
Hopefully they help you resolve what's going on with those springs. I think Hotchkis is probably one of the most reputable businesses out there for what they produce, and seem to put a lot of time/effort into doing their homework to get their products right.
Of course, any time you're making parts for 40+ year old cars there's bound to be some issues with fit. Factory tolerances on these cars weren't the best, but Hotchkis should know that and help get this figured out.