Hello, I have looked very unsuccessfully for a marine mechanical fuel pump. Yes many threads about how rare these are. I decided to go ahead and use an automotive unit. Yes, this is going under a hatch, and I really need a vapor vent line.
So, with a brand-new mechanical fuel pump I take it apart so I can drill and tap the top for a 1/8NPT. I also plug the factory vents so vapors will only come out the new vent. pictures coming soon

This is a cheap 80gph pump. I needed something that I could unbolt so I could make the top modifications and then easily bolt it back together. I have a big Holley marine mechanical pump with 1/2 npt fittings but it needs to be rebuilt.

its not a vapor line its there in the rare case of the diaphram leaking the fuel would pour into carb not the bilge

Yup absolutely correct, but most call it a vent line