I would go with 1/2 inch. 500hp doesn't need that much but it gives you room to up-grade your combo at a later date without worrying about fuel lines again.
X2....... 1/2" aluminum tube and either TIG weld or JB weld it. A Holley Black pump and -8 lines have supplied my twin 750 pumpers quite well for a number of years now.I make my own tubes
The return needs to be one size larger, not smaller.AN-10 supply and an-8 return with an aeromotive A1000 pump on my ranger. Supplies the big bad 2.9L just fine. HAHAHA!!!!!!!!!