Wow, thanks a lot for great explanation, man! I am grateful that you checked it out with a person who really knows how the stuff works. Cause I was under impression that these hoses were required only with the carbureted engines. As I was sure that modern EFIs calculate the needed amount that has to be injected and nothing remains to return. So was surprised when I was told that I've got these in my Vette.
As for the hoses, I've already ordered a kit and it's on its way to me (
https://www.carid.com/fuel-return-lines/). Yet I do not know, whether it comes with the fittings or no. So we'll see when it gets here. Anyways, I do not plan selling my Vette for now, so will keep the hose even if the OE one is fine.