1989 Chevrolet fuel pump
 

Good afternoon, My son and I are building a derby truck for an upcoming event. The truck is a 1989 Chevy truck. It has a 350 TBI engine. I am installing an external frame mounted fuel pump so we can run a single hose into a 5gal can installed in the bed of the truck (Regulation). The internal fuel pump (in tank) has three hoses on it. I only need the hose that supplies fuel to the TBI, as the external pump has an inlet and outlet. Can anyone tell me what the other hoses are for and offer any suggestions. I could just cap the others off. This truck only needs to run 8 minutes or less then it will be a memory.

I presume your external is an electric, one in, one out ?
If that's the case, then i'd get a bulkhead connector, and go into the tank about a half inch above the bottom, and just supply the pump from there. Just disconnect the internal.
Also, cant understand why you're required to carry so much fuel.

We had to remove the underbed tank and will have to install a small 5gal size tank in the bed. I can then run a hose inside this tank but what about the other two hoses. Should I cap them off?

Yes, try that, and if there's a problem, you can deal with it later.