The voltage measured at the plug gap is a function of the resistance of the ignition wires, the plugs themselves, and the cylinder pressure at that moment. If the coil you have can produce a spark across the gap under all conditions now, it is fine. Just because a large coil is rated as "able to produce 40kV" doesn't mean it will - it only produces what is needed to establish the spark.
MSD is fine to a point - if you have breaker points it is compromised. The best ignition is the 1975 B20F electronic system. Accurate, powerful under cranking conditions, and reliable. I'd have that system over MSD any day. I've measured MSD on oscilloscopes, and by 3k rpm the "multiple sparks" have become one. Lots at idle, one at max rpm. I'd far sooner have the timing accuracy of the Bosch electronic system.