Hello Bellantonio,
I would suggest .015 x 60 ft. for the G34/Flite Streak. The .015x60 can also be used on .40 size engines. You could begin with the .015 diameter lines at 65 ft. long and shorten if needed.
Seventy feet is too long and .018 diameter is not needed nor desired for the G34. The .018x70 can be used to make lines for larger engines---like the OS LA 46 or ST G51 or ST 60. Even then, 65 feet should be long enough.
Cheers,
Jim