converting points to electronic ignition
Anytime you buy replacement parts for your boat I recommend the highest quality you can afford. But you will get charged more at the marine dealer than at the local auto parts store. A distributor cap with copper contacts is the best. For points and condenser buy Blue Streak. For plug wires a good quality silicone high temperature set. Don't cheap out on parts. If you're doing the work yourself you're saving about $100/hr.
I'll never forget going to buy a replacement fuel pump for my Mercury branded Chevy engine. The marine dealer wanted $250 for a fuel pump with a length of rubber hose. The auto parts stored wanted $15 for the fuel pump and less than $1/foot of fuel hose. Guess it must cost Mercury a lot of money to package their parts.
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Pulling a distributor isn't a big deal but you'll need to line up the rotor and mark the old rotor's firing position and then install the new one in the same place. You'll also need to be inline with the oil pump drive shaft.
With the Pretronix system you can use your old coil or buy a new one from them. If you're installing an GM HEI the coil sits in the distributor. I'm not sure about the wiring requirements for an HEI distributor.