At 45 years of age, especially in an I/O to outboard conversion, I'd expect the tank to be a non-OEM replacement. My old one was 50.5"x28"x12.5, approx 75 gallons, and pretty much took up the whole space (leaving room on top for fill pipe). Something to consider is that if you're putting a new tank in it doesn't have to be exactly like the one you're replacing - mine had been foamed in, and water got in between the foam and tank and deeply pitted the aluminum - it wasn't leaking yet, but I didn't want it to keep me awake. The new one has tabs lagged to the stringers.
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1977 23' Sceptre
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