Had a similar problem a couple years ago: MSD6, Blaster2. One day it just had no fire. Coil was mounted horizontally for years. Box on front surface of rad support, wired per MSD specs, MP electronic dist.
The MSD box tested bad using the MSDD on-car test procedure.
Called the guys at Westech in NC who work on these things for Cup teams, and they were surprised it lasted that long, because mounting oil-filled coils allows them to overheat, which kills the box. They recommended an epoxy-filled coil ('91 F150 302) from any local auto parts store for $31 that's the same output as MSD's $80 coil. I shipped them my box, which they quickly repaired & shipped back by Priority, for about $90.
I had to make a bracket for the coil and also bought a $20 pigtail for a clean installation.
Problem solved, and in fact that motor starts more quickly and runs more cleanly than ever.