missing at part throttle

Poor engine or MSD box ground maybe. Yep I've cracked a plug before too (shh...don't tell anyone). What's your fuel pump regulator pressure set at? Should be no more than 7 psi, 6.5 is perfect for a Holley. I only run 7 if I'm running N20. 5 psi is too low.

What coil are you running? Are you using a ballast resistor? Shouldn't need one with the MSD. You didn't weld anything on the car with the MSD installed did you?

Are the basics good? Compression even across the cylinders?

Hope this helps.