The MSD box has a wire that goes to the ignition 'start' wire. It retards only when the starter is cranking then puts timing back up again. It can not be adjusted for more retard, its fixed
The newer msd boxes can do 25' total retard though and they too are maxed out.
I do have a switch to kill the msd spark but even that is not ideal. The engine will turn freely and as soon as that switch is turned back on you can hear the starter struggle. However, thats been my past solution on hot starts without fail.