Hey Mex, what kind of vehicle are we talking, year, make model.
Older
(+15 years ?) EFI systems pumped the fuel up front at about 60 psi. The regulator dropped it done to what was needed
(referencing manifold pressure) and sent the extra back to the tank. 2 lines. There is typically other hoses going to the tank that are for the EVAP system.
If you are going to splice into the high pressure side you really need to use nylon, high pressure fuel line and fittings. Not overly expensive, but not cheap either. The parts might by hard to find the parts where you are.
Watch this video
Now to repair rusted fuel line. Ignore that funky tool ! Boiling water will soften the nylon line enough that you can push the fittings on by hand.
OR
Installing fuel line fittings without heat