Do you have spark coming directly off the coil? 5.5V for cranking seems low. Your resistor may be bad or the coil itself may be bad. IIRC, the 6.3 has the same spark setup as the 3.5 and 4.5, meaning one resistor is bypassed during cranking. Pretty sure the coil, wires, cap & rotor are the same as those engines as well.

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2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

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1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
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