Double check your return lines. Sounds like air intrusion to me too.
1) Did you buy a new return line kit with o-rings and caps? Or did you just try replacing the hoses in between? (Always do the kit.) Let us know.
2) Did you lube up the orings with ATF, Diesel, or some o-ring grease? Maybe one pinched or got cut. They are a little tricky but "pop" on when you put them on the injector if done right. Let us know.
3) Open all your injectors lines at the injector. Run the starter for 10 seconds on 20 seconds off. Get all the air out! Then re-tighten and try again. Mabye you still have some air trapped somewhere and you are close to it running.
You already had a air issue so maybe while working you bumped on something and made it worse. So after all this chase backwards to the tank selector valve and from there to the fuel tanks. If both tanks have the same issue, it is likely between the engine and the tank selector valve. (VS both tanks have leaks between the selector valve and tank.)
Still stalling, next replace the fuel filter.(You said you saw water in it, you did not mention replacing it. Perhaps you have. If so make sure it got screwed on tight. Always fill it with ATF or diesel before you put it back on. (More air you don't need to deal with.) Perhaps the fuel seperator or lines to it are causing an issue. Many folks have challenges with these.
Then check your lift pump is working. Hook to the shrader valve on the fuel pump and watch the PSI while running. You should have at around 5 psi. Anyting under 3psi and you likely have a bad/weak lift pump or a blockage to or from it. If you have good PSI you may want to replace the rubber hose from the fuel line to the lift pump. Use new clamps. That will insure you do not have a leak there.
Could be you just had a mishap when doing the return lines. As you said it was running before (Just hard starting after sitting for a while.), now it only runs for a short time after doing the lines and looking at the fuel filter. Could be you still have some air you need to get out of the lines.
After you get it running you can address the dash gauges. The stock dash gauges and sensors are not very good or reliable. Advice from this group is always put in a second set of 3rd party gauges, you then will see the problem before it hurts your engine. Oil pressure, water temp, and voltage. Add EGT and boost if running a turbo.
Let us know what you found.