Changing fuel system

Getting ready to upgrade my fuel system to along with my engine. Couple of questions though.

1. What level should I get the gas tank down to before I take the sending unit out? Going to try and change without dropping the tank or maybe just lowering it. I was thinking at least half a tank but I don't know. I just took the car out today and forgot I filled it for winter already so I guess I'm going to be driving it around to use some gas up.

2. How can I prime the new system to get ready for the new motor. I'm changing to a 3/8 line which wont hook up to the stock /6 pump right? So how can I get the gas through the new line and ready for the new motor's first start?