MSD 6al... (weak spark)

Coils do not have a ground, and MSD works completely different from "conventional" ignition, that is, breaker points, Mopar breakerless, or GM HEI based systems

MSD works more like a photoflash. It charges up a Great Big high voltage capacitor with several hundred volts and when triggered, sends that pulse through the coil.

(By the way, THE ONLY wires of any kind you should have hooked to the coil, are the ones coming from the MSD box. No radio condenser, no tach, nothing else.)

So far as a "coil ground," with breaker points, the spark happens when the points open, so you have a series circuit there, from the coil primary, through the (distributor) "condenser" (capacitor) to ground.

The MSD may ground the coil by means of the coil NEG wire which comes from the MSD, that is, the pulse would be applied across the coil NEG and POS terminals, the NEG being grounded by the MSD internal wiring.

One other thing. How exactly, and I do mean EXACTLY, are you checking spark?

You may have a bad (internally burned) coil wire?