Re: 1965 Chrysler / West bend 50
To be 100% sure as my motor is a 66 "Chrysler" motor and that chry may have done some electrical changes,
Take the cable off your starter, Using booster cables and a charged battery, attache the ground - cable to the block of the motor on a paint free surface, then touch the positive+ cable to the starters power stud. if the motor turns over you have a neg ground. if the starter spins but does not engage the fly wheel, reverse the cables....if the starter now turns over the motor you have a pos ground.
Doing this test will not harm the other electrical parts and a starter is simply a DC motor and just wont engage if it is spinning the wrong way. after this you will know 100% what ground your motor needs. Let me know results.
As for putting a radio in, if you have a fiberglass boat it is not a problem because there is no electrical bond between motor and hull.
To be extra safe you can always make sure their will be no problem by mounting the radio with wood or plastic, and just run wires from the bat to the radio, an on/off switch to the switched power of the radio is always a good idea too to reduce unwanted power draw when the boat is not in use. I hope this makes sense. If you have an alum boat, just be sure to mount the radio in a isolating structure of some sort so that the radio chassis does not come into contact with any of the boats metal.....same with the antenna.
My Motor is - Ground, from the factory.