I just replaced the flexible piece from the fuel damper-above the fuel pump, to the steel fuel line and it took 5/16 inside diameter hose...so the steel line is 5/16 outside diameter. Hope this enough info to get your leak fixed.
The fuel line at all locations IS 5/16" except for the main feed off of the fuel tank, which is 12mm. If you need hose for the main feed, goto a truck stop/ service center as they can sell you what you need by the foot for cheap(I paid $1.35 per for mine). BTW, there were numerous factory recalls on the F.I. lines in the '70's on these lines in between the pipes, and service reps recommend the awfully complicated and time consuming task of replacing all of these lines every 25K or so. The heat from the exhaust manifold kills 'em.