Not a bad idea, in some ways, I'm in a weird place at the moment where I have so much work that buying a new one for just under $500 would take me less time that fabbing one in that manner. If it was an NLA item, that is an attractive idea. I worked at a fiberglass boat factory in my early 20s, I understand the principle but never thought about using the old tank as a plug. Also, one wonders if it would be just enough larger to be a PITA to refit.

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1986 300SDL, 362K
1984 300D, 138K