I have a 2000 chevy express 1 ton with a 5.7 engine with 345,000
miles on it. The van was running just fine when it quit. I thought it
was the fuel pump so I replaced the fuel pump, fuel filter and fuel pump
relay. The spark plugs, wires, cap and rotor were replaced about 50,000
miles ago, all look very good still. I normally go 100,000 miles before
changing those items.What is happening now is the engine starts for
about 2 or 3 seconds and quits it will do this over and over. I tried
keeping the key in the start position, but it still won't continue to run.
Any help you can give me would be greatly appreciated. Thanks Bill
PS I have owned this van since it was new and have only replaced the
item listed above and the water pump, pcv, air filters, belts, brakes,
and so on, you know standard ware items.

It still sounds like a fuel issue. It is possible the new pump was DOA out of the box. Common issue with rebuilts. Can you borrow an OBD II code reader and retieve any stored codes? Can you get a fuel pressure gauge on it?

Fuel pump is working you can hear it running and it has good pressure.

But how much pressure.
Post back pressure readings?
Does it have good hot blue spark to plugs?