Heh, heh... I was using 40x17 on track-bike, good for 38-39mph at CSUD.
Ok, so you'll need lower gearing than 42x17 = 2.47
That rules out the 39x15 as it's taller at 2.60, leaving you with 39x17, one out from middle.
Measure offset from centre for chainline, don't just eye-ball it. Easy way is to measure from drop-out inner-face to centre of cog, call this
X
Then divide OLD by 2 and subtract X. For example
(130/2) - X = 65 - 20 =
45mm
Take ruler and start at centre of seat-tube and measure actual number you calculated out over crankset. Which chainring is closest to number you calculated?