This one is a bit hard to answer specifically Malcolm, but I can generalise a bit.
The spark plug gap can be influenced by a number of factors maximum gas pressure in the cylinder at firing being one of them, but the more basic issue is the output voltage of the coil. This itself is influenced by the ability of the coil to fully charge before it is discharged during firing. A single V8 coil charges and discharges 4 times per engine revolution.
Plug gaps back in the good old (not) points days were often 25-30 thou. As coils got better and amplified electronic triggers took over plug gaps generally grew to 30-40 thou. I would think the setup you are talking about is likely to be in this range more likely 40 than 30 if your coil is a good one.
The later multi coil systems make life easier for the coil because each coil typically will fire once per engine revolution, so they get better outputs. My current road car has one of these system and a 60 thou plug gap.
One thing I am unsure of is the multi spark effect of the MSD. My understanding of their effect is to get the coil to fire multiple times starting at the normal firing point. I think it is possible that this could make each spark weaker due to the reduced time the coil gets to charge up. It is possible that this device reduces the gap the coil can support.
The bottom line is that my GUESS for your plug gap is 35-40 thou. The coil manufacturer may be able to help if you can find them.
Sorry I cannot be more specific than that.
Regards
John
