Take the rear shackles off, get new rubber bushings for them, loosen the u bolts then push on the shackle end of the springs & measure the side to side room, tighten u bolts, reinstall shackles but DO not over tighten them like most people do, I did this but I have sliders in place of my Shackles worked great.

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1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5