MSD Ignition Question

I wouldn't bother with an external box with that. Not in your application. I'd only upgrade to adding the box if you are having problems with it misfiring. And unless you're running a lot more gear or compression than makes sense for a cruiser, I doubt you're going to run up against the Ready to Run distributor's limits.

The regular MSD box only gives multiple sparks because its sparks are a lot shorter. They're hotter, they're better at firing fouled plugs, but the individual sparks don't give as long a burn, and that seems to cause trouble at low RPM. So they added multiple sparks, pretty much to tame a high RPM racing ignition and make it work at low speeds too. Though I'd imagine it might "re-strike" misfires as well and have a slight advantage. But probably not enough to pay for itself in gas mileage.