just my 2 cents...
I replaced the original tank on my 84. The original setup was foamed in. PVC passing water from bow to bilge, bypassing the tank compartment and tank hatch sealed with silicone. This setup lasted 25 years. So I stopped pondering what to do and just replaced exactly the same way.
If you are foaming in then definitely make sure no water can enter the tank compartment from bilge or hatch.
And I agree that the power washer method was "best" and that wasn't very easy either. Be careful to find all the little pieces of foam that are now everywhere in your bilge.
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