Here is a picture of your fuel system on the 300D:

Number 11 is the stepped fuel line that you are asking about; I believe one end is 8mm and the other is 10mm. Part # 1114761526 so you can order it if you wish from Pelican Parts.

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Marc-André

W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car