Engine has no power and it stalls out?

When you changed the fuel pump out did you use an AC Delco unit? If not let's check the fuel pressure because it sounds like the fuel pressure is low or you have a clogged exhaust system. These guides should help us figure out what's going on:

https://www.2carpros.com/articles/engine-has-low-power-output

and

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

and

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Please run down these guides and report back.