my cousin has a 454 in a 89 chevy 3/4 ton. the problem is that it starts fine but it dies when you step on the gas. any ideas what causes this?

it did have some old gas in it however the gas guage was very low. it was then filled with premium fuel. so therefore gas shouldn't be a problem.

Sounds likely to be a bad TPS or plugged injectors ('89s were TBI systems). An "old school" thought might be that a vacuum line is broken and when the pedal get pushed too much air finds its way to the intake, stalling the engine. What happens when it goes from Park to Drive without hitting the gas? If it stalls then as well you can eliminate the TPS being the problem.

The fact that little gas was in the tank is irrelevant if it sat for years- it's the gas in the fuel lines and TBI unit that matter, not the gas in the tank- if that started to gel up you'll have problems.