I had ordered the kit but even though Chris said #6 would be fine (634hp) every other source said #8. So I am going with a larger pump. You have to assume there will be some degridation in flow as the pump wears and the filter starts to clog.
Also use the same size return / feed, under idle conditions you'll be sending most of the fuel right back to the tank. Under high vacuum the regulator is designed to lower the pressure to 40psi so you don't want any restriction in the return.
Basically, having larger lines won't hurt. The price difference is not very much. I'm also running a y-block with two #6 lines into the rails and two #6 lines to the Reg. This will help keep the pressure equal across the rails.
Might be overkill, but so is everthing else about the car.
