I can only tell you what the pros did at my house; soft copper from the
tank to the house, then 3/4" black metal pipe throughout the house. A
couple of things I picked up talking to / watching them; Don't use
galvanized pipe, it flakes off and plugs pilots and such, leave a drop
( a T with a nipple and cap facing down) every so often to catch the
liquid and leave it a place to evaporate, be very paranoid about every
connection & use gas approved dope.