T, I'm not running a 393 but I am running all MSD ignition.  Dizzy, 6al and Blaster coil.  I am running E3s not sure I can tell any difference but have not had any problems.

Also I sent you an email to your sbc address!!

To avoid changing too many things at once, I would run whatever plug you are currently running when you switch to MSD and start there.  If it was in the ballpark heat range wise, there is no real benefit in changing - if it ain't broke don't fix it.  I found a few items that may be of interest - the one below is probably the most useful on plug type from MSD tech -I can't give a specific recommendation for your application as every application will be different.

What I can recommend is that you use a good quality normal copper plug, staying away from any type of speciality or 'hype' plug such as platinim or iridium. Pick a plug heat range that might be a little colder than what you would expect by 1-2 heat ranges. 

As far as gap goes, start at about .035 and bump the gap up in .005 increments until the engine idles smoothly and when reading the plug you show a nice 'clean' burn; not too rich not too lean, with a nice carmel /paperbag color on the porcelain of the plug.

From what I have read over the years. MSD systems SOMETIMES have problems with platinum plugs due to the size of the conductor.  Your results will vary but when in doubt, follow the recommendations of the manufacturer - they often know how their own systems work best.

I can't seem to find your link any longer but when I looked at it the other day your motor seems to lay down in the higher RPM ranges.  Your carb may be a bit small for the application or you may be running too stiff a spring on your vac. secondary which is preventing it from fully opening (if it is vac.).  Your A/F ratio looked pretty good and if the motor performs the way you like, ignore my observation.  Exhaust, intake, cam, and heads also have an influence on torque and horsepower and the carb may actually be well suited to the total tune of your motor.  

Terry G......I have the MSD box and coil with a duraspark distrib.
I run what ever plug is on sale!
 I change just "whenever" I get in the mood. They last a loooooooong time with that MSD firing them
 That MSD fires such a big/hot spark almost anything with an electrode will fire!
MSD recommends bigger gaps on the plugs too. Can't remember how much butt.....I think like 45 or 50.  Did you know AC plugs were made by the guy that invented Champion plugs. Same tech....just different name.
6sal6