I am a strong believer in switching hot. This keeps the energized portion of a circuit in a smaller area which may help prevent problems if wires chafe or some other similar action. Also, I like to keep to one standard when wiring thing... helps me remember how I did it years down the road
Regarding the engine, I could be wrong but I don't remember marinized Iron Dukes, just the 3.0s were marinized. I had an Iron Duke in my company car in 1985, a Chevy Celebrity Station Wagon, 90,000 trouble free miles in three years
I agree on switching hot. And wiring is critical. If the hull and engine are sound, it will last a long time. Take the time to do the wiring right. You're not doing this for resale value, you're doing it because you want it to be reliable and repairable when required. Do it right, do it once.
This is what proper boat wiring can/should look like: