I have the same size diameter feed and drain lines clone has, with the CB header that you are looking at. Works great for me. As Pete mentioned...clock the turbo drain VERTICAL for best results. I haven't had a oil drain issue with my set up (CB Header, oil drain to deep sump).
As for the header diameter, I asked the same question when I was building my motor. The difference between the 5/8" and 3/8" will not be noticeable with this type of turbo setup. There was a brief conversation regarding it in my build thread.