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.