Does the Duraspark have a pin to add some spark retard like certain GM HEI modules do? If you really are supposed to set base timing to 20deg such a thing may help with hard starting.
There are a few other things in that DML link that give pause and I would probably call Don to confirm that base advance figure.
FWIW I've tested at least 13 degrees of electronic spark retard without phasing issues - spark falling off, jumping posts, or the like - and it seems like there is room for a touch more. Exhaust will start to get hot, tho