In 1:1, you'd probably be looking at -6 or -8 AN hard-lines for fuel delivery. Probably -8 for a funny car and larger for alcohol. Dash 8 line is nominal 1/2 inch ID, so about 9/16 OD. 9/16" fractional is 0.562 decimal. Divide by 25, you get 0.0225. So, .022 stainless or nickel wire would look correct for a dash 8 (-8) hard line in 1/25 scale.
.022 stainless wire is commonly used for fishing lures and leaders, and also as safety wire in aviation (as well as many other sizes, free if you know an aircraft mechanic). There is .022 nickel welding wire. There's also steel music wire (piano wire...you may be able to get short pieces free if you know a piano tuner). Or, you can use brass and color it, as Greg suggested. Hardware and automotive stores often have silver-colored cheap tie-wire in the correct diameter too.