At first, I intended to completely replace the old rusty steel fuel lines on the truck in question with GATES rubber, PLUS a third line to handle the regulator's return.

Truck in question:

https://www.dieseltruckresource.com/...p-t319084.html

I was going to use 3/8" for everything; it is the same price as 5/16 or 1/4 and I already have plenty, plus adaptor barbs.

Then I got to thinking, seeing as the draw-line is going to be a single 3/8" line, and the factory return is a tiny 1/4" line (actually a bit smaller, as the steel tube fits inside the 1/4" hose), why cannot I TEE the factory return into a single 3/8" return-line in the location where the factory return hose connects to the factory steel line, and let this single 3/8" hose carry both the injector-pump return and the regulator return back to the tank ??

Thus, I would eliminate the need to bulk-head the extra return-line into the tank.

Is there any danger that the regulator's return could over-pressurize the injector-pumps return and cause problems ??

My line of reasoning is that the lift-pump can draw no more fuel through a 3/8" line than another 3/8" line can carry.

Wisdom please.

Thanks.