Noodled this overnight and it makes sense- the four lines from the tank vent vapor to the collector in the trunk, which then collects it (from the highest point) and routes it to the thingy on the valve cover (which I then have the wrong one of). The net effect is to collect vapor from the four corners of the tank and feed it to the carburetor(s).

Think I figured it out- this car was built as a CA-spec 318, so no vapor separator above the fuel pump and no fuel return line going back to the tank. It would have only the fuel line and the smaller line going from the valve cover spigot to the CAS thingy in the trunk that is attached by four vents to the tank. I pulled the valve cover spigot from the 340 I took out and indeed it has three ports. The hard line probably angled up when it entered the engine bay to get to the valve cover, and I must have bent it down in the direction of the fuel pump vapor separator before I installed the Hemi.

But of course the 340 wasn't original either, so who knows how it was hooked up.

The old fuel tank didn't have a return line attached to it, so that was a red herring. But the spigot is there, and thankfully the new tank has one too.

So the thing to do is run a third line from the fuel pump vapor separator to the tank, and attach the vent line to a port leading the carb- I'll probably tee it in with the PCV hose, since I don't have a three-port spigot to fit a Hemi valve cover.

Per the shop manual the fuel pump vapor separator is intended to help prevent vapor lock. But if things aren't hot enough to vaporize the fuel, the separator fills with liquid and the pump shoves some of it through the vapor orifice back to the tank. Disconnect the vapor outlet and crank the engine and you get gas, gas, gas. At least I did!

Thanks to all for the input.