Which fuel rail be better the 99-02 or the 03-07 fuel rails? for 5.3 swap in 91 silverado.

 

Return style would be easier for your conversion. That way you only need to upgrade the intank pump, then everything from the tank to the engine bay you can leave stock. Custom lines from the hardline fittings to your intake and the system is done

To go returnless you'd have to get creative

 

It would look lot cleaner with return less