Spark Plugs for Slant Six?

No, that would be a bad idea. Run 0.040" to 0.045" and make certain you're using good high-end plug wires and a good cap and rotor, which is difficult; read and follow the suggestions in this long (but worthwhile) thread.

(Also, MSD is a great way to get stuck somewhere due to ignition failure, someday, without warning. You're much better off, both result-wise and money-wise, doing the HEI upgrade instead.)

Thanks for the info.

I'm going to stick with it because it's already on the car and functioning fine. I know adding more technology adds failure points but thats ok. I'm also running a super sniper EFI which I get told all the time I shouldn't have complicated things.