Being the proud owner of a pair of Qs, a lefty and a righty, I have been following along with some interest. This post has caught my eye, and now I'll pitch my 2 cents in.
Derek, you must never use car parts on a boat engine. Yes, marine parts are more expensive, and along with gaskets and such you can also use automotive sparkplugs and ignition points/condensers. The automotive versions will rust faster, but the engine will not know, or care, and they are the kind of things you replace every year anyway.
The automotive parts you can't use are;
Fuel pumps, car fuel pumps have a weep hole in the bottom of them. If the diaphram busts gas leaks onto the ground under the car. Marine fuel pumps do not have this hole, when they bust a diaphram the gas leaks back into the crankcase. This is why you check the oil before you go out and after you get where you are going.
Carburators, same deal, automotive versions have vents in the top of the float bowels to let excess gas out of the top if the float needle sticks, marine carbs are vented into the thorttle bores, for the same reason that marine fuel pumps have the internal drain. The idea is to keep raw gas in the engine, and out of the bilge. It is a lot harder to blow the boat up if there is not gas in the bilge.
Starters and alternators for marine service are spark protected. Car parts are not. The marine versions have spark arresting screens, different brush material, and some makes of alternators have gasketing between the case parts and bolt on regulators that the car parts don't have. Don't let anyone tell you that a starter is a starter, and an alternator is an alternator it aint so.
All distributer caps are vented, if you look a new one for you car you will find a small hole near the base of the cap, that is the vent. What you won't find on the car version is the small spark arrestor behind the hole. By the way, Not all marine caps have a spark arrestor, on the ones that don't (like a Mallory flat cap) the way the distributer housing is constructed when the cap is on, the ridge in the housing serves as the spark arrestor.
The idea is to contain stray sparks that could happen. Stray spark+hot engine+slight fuel vapor from hot engine=BOOM.
Yeah sure, I know we all run the blowers for 5 minutes after refueling, especially when there are 10 boats waiting for your spot at the fuel dock, and the dock guy is telling you to get going, but better safe than boom.