"One-way" fuel line valve..?

No, and a 198 Slant just sips fuel. I even run that set up on a 472 Dual Quad Hemi. But I will say with all 8 barrels open it needs the electric pump on feeding the mechanical pump.

As the slant-six fuel only sips fuel, is it possible to install the electric pump just ahead of the mechanical pump, on the rubber inlet hose? It's a lot easier than near the gas tank, especially for the power wiring.