if a valve was stuck, and spark was present, the engine would backfire or catch on fire. your spark is going away under compression or for some other reason. Keep a fire extinguisher close by
Gee I went out with a friend in his 18 footer with a 3.0 MerCruiser. It's vibration reminded me of those 90's 4 cylinder inline motors in small cars. It handled the boat OK. I had a similar boat with s 4.3LX. It was much smoother and much more powerful.
Gee the first carb I saw clogged with varnish was in the fifty's. This was way before adding alcohol to gas was thought of. we also needed to add dry gas alcohol) to fuel tanks to absorb the water in winter. Today's fuel with alcohol eliminates a lot of that.
You might look up the weight of the V6 and V8 and see the difference. I expect it to be nominal. You might ask the experts if the motor mounts will match
Here is a carb repair kit with lots of extra stuff. Springs, jets and other stuff. the price is extra as well. BTW, Gasoline with alcohol keeps water out of the fuel and keeps carbs clean, unlike what the wives tales say...
New Jersey usually gets two weeks of extreme heat and extreme cold (by NJ standards) each year. Here at the shore that translates to about 5-7 days of whole house air conditioning per year. Obviously, the heat needs to be used from Nov until Mar/April.
I looked up the plug wires from a '75 Merc 20. https://www.crowleymarine.com/mercury/oem-parts/outboard/1975/1200205/flywheel-ignition-coil-stator-and-switch-box-group-no-15?ref=28
parts for a '71 were not listed.
Those plug wires look to be push in to the coil, with spark plug boots and...