[MT-2500]

What year and engine?
Airtex fuel pumps are on the bottom of the barell junk list.
You are on the right track about AC-delco?delphi fuel pump.
The dealer pump is Ac-Delco/Delphi but at full dealer price.
They are.
Around 300 US dollars at a good aftermarket parts place.

Check Carquest and AZ and O'reilly And other parts places may be able to get them and tell them you want a good AC-Delco/delphi pump not a 90 day wander or Junk airtex pump.
They also have or can get the fuel pump retaining ring.
If vent valve is broke get another one or if just unplugged plug it back up.

But to be sure it is the fuel pump run the full fuel pressure test.
May be to late if the tank is down.
But should have been done when the repair shop checked it.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.