My guess is that 0.5 ohm is close enough, what Mopar requires, and you might get another 0.3 ohm from the wiring and connectors anyway. MSD's instructions appear generic, for many engines. I would run it and see if the coil gets too hot. If so, it will start missing at high throttle, in my experience (ran without a ballast once).
The Blaster 2 is designed for later ignitions, like GM HEI, that don't require a ballast. Using it with the old Mopar electronic ignition w/ ballast is a waste. I have a coil from a ~80's Mopar (Ram Van) that says "for use with electronic ignition". I recall the same engine had the Gold box and a ballast resistor. Maybe that would be better for you.
Is this a new conversion kit? If so, why are they providing the earliest dual ballast design? Most all ignition modules you get from the store today don't even use the 5 ohm ECU resistor, even those that have 5 pins (usually a dummy). It would have been simpler, cheaper, and better performance to install an 8-pin HEI module w/ coil as I did in my slant, as have others. Then you don't have to worry about matching a ballast to the coil. If you don't want a junkyard trip, TrailBeast sells the components new for ~$100.