That right, no fuel pressure....
If you had no fuel pressure, you'd get no spray out of the Schrader valve.
Therefore, you have at least SOME fuel pressure. Get a pressure test gauge, find out if you have ENOUGH pressure, and if you have pressure all the time you're supposed to.
And there's those terms again. I thought an engine had either a throttle body, OR injectors.
You have both throttle body and injectors. When the injectors are integrated into the throttle body, you have Throttle Body Injection (TBI). But you're saying you have a 2000 model year Vortec 350, which has a dry throttle body (no fuel) with eight separate injectors each spraying directly into the individual intake ports.
The original Vortec injector system uses a bigass lump of an injector system with eight tubes leading to spring-loaded individual injectors--poppet-style injectors. This can be upgraded to an injection system that uses electronic injectors which works better and is less-sensitive to fuel pressure.
SUSPICION: You have fuel pressure, but not enough to overcome the spring loading of your old, poppet-style injectors. Engine cranks, doesn't run. Has enough pressure to spray fuel out the Schrader valve.
POTENTIAL SOLUTIONS:
1. Replace fuel pump, repair damaged wiring, and/or replace clogged fuel filter so that fuel pressure improves enough to fire the poppet injectors.
2. Replace fuel pump, repair damaged wiring, and/or replace clogged fuel filter so that fuel pressure improves enough to fire the poppet injectors. AND replace poppet injectors with more-sophisticated electronic injector system as sold by several suppliers--ACDelco, Standard Motor Products, Dorman, and probably others.
I don't have separate injectors for each cylinder,
Your truck was built with separate injectors (nozzles), but they're all tied to the main body of the "spider".
I heard they didn't work very well and saw the "spider" replacement.
There's two kinds of replacement "spiders"--the original style that uses the spring loaded poppets, and the newer/better style with electronic injectors. When they're not defective, both systems work. The poppet style is more-sensitive to fuel pressure problems.
Since it runs by putting gas straight into the throttle body, I've got to assume so fuses are good?
No. Your engine runs when you pour gas into the throttle body, but that does not mean that the injector fuse(s) are good. But I have to be careful here, I haven't studied the wiring schematic for a Vortec 350. I'm making an assumption that the injectors have a separate fuse or two.
A friend brought a fuel pump relay over to try, because mine is unique under my hood, and no changes. I've never dug into 1, but I was told there are 2 injectors in, or under the throttle body.
A Throttle-body injection system for a 350 has two injectors. Has your truck been modified from Vortec style with 8 injectors in a spider system to 2 injectors on a throttle-body system? That would require a lot of work, and would be a step BACKWARD in power and fuel economy.
I WANT PHOTOS OF THIS ENGINE. IT'S TIME TO SEE WHAT WE'RE WORKING WITH.
And the FPR (fuel pressure regulator) stays open and slows the extra fuel to return to the tank so it can be used by the engine.... And if that's all that's on that fuel line/system, it's acting like the FPR is completely closed off? Nothing getting threw the injectors in the throttle body, but no movement allowed at all, unless I depress the fuel pressure check valve, which looks like it just has nowhere TO go. Like blowing into a pipe that has the end closed tight. There's no movement, or pressure unless I open a path for it to move up there. Then it's only to the valve....
No. That is not the way things work.
I don't want to take time to explain Vortec vs. TBI fuel plumbing theory. Post a photo or two of the top of the engine so I can tell what system you have. I haven't made a big point about your troubles with the fuel pump; and maybe I should have before now. Truthfully, I made the assumption that some moron at the parts store sold you the wrong stuff--but--maybe someone has severely dicked with your truck, and that's why proper parts don't fit right.
POST PHOTO(S) of the engine.