Msd upgrade!!

HI everyone I am looking to do an upgrade to a 1969 evinrude 115 which has an automotive type sensor pickup, coil, cdi box and distributor system on it stock. I would like to replace the stock cdi box which is bad with a msd 5520 I know this is a non marine unit but I can install in on the boat where there should be no chance of the unit seeing any water unless the boat goes down. What I am looking to do though is drop the voltage of the unit from 435 volts down to around the stock units 300 volts which will allow me to get the voltage going to my distributor around the 25000 to 3000 volts mark I need. Can anyone offer a suggested way of doing this. I was thinking a ballast resistor between the box and coil but I am not sure what resistor to use. The other thought I had was I have found some 73:1 coils that in theory would drop the voltage to max of around 32000k. Has anyone done this or can anyone offer some input?