Sounds like you are on the right track to me....Flexible tube for the oil change is a good idea, some of the folks here have installed valves in place of their drain plugs so that the oil change is a much simpler and cleaner process.
As far as oil change or tune up goes, I tend to take the approach that these 155 v6's are really just JEEP motors with marine parts on them. For oil I just use a nice quality 10/ w30 and a can of engine restore (I have the origional motor in mine) If yours is the boat I think it is you have a 231 even fire, don't you?
Anyway, fresh plugs and wires (great), oil change (your on it), maybe a new fuel filter/ water separator ( I change mine about every other year), Check and set your timing (mine LOVES this), adjust your carburetor idle mixture, adjust your idle speed then repeat til everything is just right and she'll run like a treat.
I use a vacuum guage to set my idle mixture...it really takes the guess work out of adjusting the carb. Like I mentioned earlier, using the vacuum guage, I set the timing to 6 degrees BTC, set the idle speed (on the linkage port side of carb) to about 750-800 RPM, adjust the idle mixture screws (one at a time) till they are at max vacuum (first peak), then check everything again and adjust until all those parameters are within spec....then enjoy your smooth running boat!
Here is a link that talks about general automotive tuning with the vacuum guage...this directly translated to tuning your boat (for the most part) you will not have a vacuum advance or a brake booster
http://www.tuningmadeeasy.com/tuning/va ... nder-tool/Cheers,
B.C.