You need to re-read the instructions because that is NOT what they "say."
MSD uses a "trigger" wire which causes the box to power up, this is the "small red," and draws little current. As someone mentioned above, your original switched "ignition run" and the brown "bypass wire" need to be hooked together, in other words, all wires originall hooked to the ballast need to be jumpered/ junctioned together. THEN your original coil + can go off to the small red trigger wire.
The "main" box power --the wire which actually powers the box is the large gauge red wire, and is supposed to be hooked to a good big live power source such as the start relay. In the chart below (out of the destructions) it's called "HEAVY RED"
If you are getting LESS THAN 10V during cranking, either you have wiring issues, wired wrong, bad / dead battery, or your meter is AFU. A good battery during cranking should deliver closer to 11V.
The destructions:
http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions.pdf