You've got a pretty good understanding , now.

Verify, with a sight glass, of good, solid fuel going into the engine( at the transfer pump=changes from suction TO pressure), so now the fuel travel thru secondary fuel filter, then into rear of cylinder head, fuel goes thru a passage in the head, the fuel passes by/supplies #6 injector, then #5, then #4, then #3, then #2, then #1 injector, then out to the fuel pressure regulator, then back to the fuel tank.

We put sight glasses at transfer pump(=supply) and one at the elbow, right AFTER the fuel pressure regulator(at the very front, top, left side of the engine). This is the fuel return line(=return).

Changing "coppers" is a big deal. You must have the right tooling! My guess is that the dealer you are going to, doesn't have it, or if they do, they may not know how to correctly use it.
PLUS, now there is newer, faster tooling available(highly recommend it).

BUT you MAY NOT need coppers, you may just need your "coppers" reamed(new cone-shape cut into them, at the bottom), then reseal and reinstall your original injectors(with newer tooling, that forces then into the "cone" of the newly reamed copper). PLUS all the adjustments