Reading your description of symptoms, I immediately thought fuel. I have seen a bad coil give similar symptoms, but you have replaced it.
If the ignition switch is 43 years old, then I'd guess the fuel tank is that old too. While you have replaced the fuel, that doesn't mean there isn't sludge in the tank that interferes with fuel pick up. When you used less fuel, at idle, below 1500 rpm etc, everything is fine. When you accelerate, the fuel that is in line runs the engine OK, but then, reduced fuel flow causes the engine to bog down. When you reduce demand (slow down) the clogged fuel system can keep up with less demand. I agree with the thought that an electric fuel pump could exhibit some of these symptoms if it isn't getting steady electric feed, but not my first suspicion. Either way, it sounds like a fuel problem, not ignition. In a car, I've seen a clogged air filter give similar problems, but the flame arrestor on a boat should be OK in this regard. Is there a safe way to use an alternate fuel supply and not draw from the built in tank?
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