Anyone run two factory bent/repro 3/8" hard lines for your EFI supply and return lines? I imagine there would be interference since they're trying to turn in the exact same place. Perhaps use a factory 3/8", and replicate the factory1/4" return in 3/8" manually.

Fine Lines would probably bend you up a set like that if you asked them.

Generally, you can go a size down for the return. Any particular reason you need that large a return?

If you put the pump and regulator setup inside the fuel pump then you only need one fuel line. That is the way most of the modern cars are plumbed.

Fixed

Another option that might work

https://aeromotiveinc.com/aeromotive-corvette-filter-regulator-combo/

That is what I did on my car (E-Body), just have to rebend them slightly here and there to fit. I used the 3/8 Stainless lines from Inline Tube. I prefer steel hardlines to rubber for the long runs under the car.

That is correct. Looks like I mixed up my first post.

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Have used those on several efi systems I have installed. Have never had any negative issues with them.

Which EFI are you looking at?

Holley Sniper. From their instructions:
"We recommend at least a 3/8” feed line and 3/8” return line."
"The Sniper EFI system requires a return fuel line to the fuel tank."
"The return line must not present a pressure restriction to the return fuel flow. There should never be more than approximately 3 PSI of pressure in the return line. A line that is too small, or has restrictions will cause tuning problems with the system."

Now, I watched one of their videos that says their in tank pump has a built in return, so an external regulator and return is not required. Waiting on a response from them to clarify is this pump, with the Sniper, still requires a return line or not.

https://youtu.be/n8Avatc3fd4

Ah, ok thanks for the clarification. When I put a Holly EFI (Projection) setup on my Diplomat it specified a 5/16" return and a 3/8" supply. But that was years ago.

The Sniper doesn't need a return line if the regulator is in the tank. There is nothing to return if the Sniper receives 60 psi fuel. In that case you just plug the regulator out port on the back of the Sniper and you're good to go.

Ive done this on two cars. Like everything else I need another sending unit pump combo that is back ordered for a 69 B-body

I will also mention that one big advantage of utilizing a return line is the fuel not being used recirculates back to the tank
(constantly moving) thereby keeping the fuel cooler and less prone to vapor locking which really shouldn't happen at 40-60 PSI

I'm using 3/8 supply, 5/16" return pre-bent stainless steel lines from Fine Lines.
Having to slightly tweek the bends is not a problem, heck they come bent in the box and you have the straighten them anyway.

I think if your using a 350 lph or smaller pump, the smaller return line should not be an issue. I think the return port in the built-in regulator is less than 1/4"?

The stock Mopar fuel lines are either 5/16 outside diameter or 3/8 O.D.. The inside diameter is much smaller, especially going around 90-degree corners: scope:
I like and use 1/2 inch aluminum O.D. tubing for my performance cars

Cab, 1/2" for EFI? I mean you can but that will support a huge amount of power

That sounds good in theory but in reality it eventually heats up all the fuel. Probably not as bad with a sniper as with two fuel rails but it still will take some heat back to the tank. My car with port EFI will heat the fuel up to 120 degrees pretty easy, I even have a cooler with a fan on the return line. I may eventually move my regulator back by the tank to prevent this but for now it is what it is. I have seen fuel temps as high as 140* in heavy traffic(I have a flex fuel sensor that also reads fuel temp).

N/A with mechanical fuel pumps only for me up to now
EFI soon