as in the MSnS Extra Ignition Manual. You need to change your dwell from the 3 settings (cranking, running, minimum)when using a single coil, to a fixed dwell at 50% when using the MSD 6A
I did this today and the car revs, and pulls hard at 20lbs of boost.
this is single coil
http://www.msextra.com/manuals/MS_Extra_Ignition_Hardware_Manual.htm#singlecoil
this is msns1 v3.0 with MSD
http://www.msextra.com/manuals/MS_Extra_Ignition_Hardware_Manual.htm#msdout
it looks like you must change your dwell settings!