18 AWg wire is bigger than you need and MIGHT cause the end to whip around in the wind a tad more but not significantly. Make a good solder connection and then put a short (I said SHORT) piece of heat shrink tubing over the splice. A Monokote heat gun should have enough heat to shrink the tubing, if not then carefully use a match. Be careful not to damage the wiring insulation. The more individual strands a wire has (for a given gage and assuming the same metalurgy) the more flexable it will be. You do not want stiff wire or a stiff splice as this will cause a fatigue stress failure point. This is why most recommend total replacement when you can. The original wire is probably around 22 AWG and maybe 24 AWG.

NEVER use single-stranded (solid) conductor wire where vibrations occur unless it is well supported and clamped in place.

BTW - The difference in optimum antenna lengths from one end of the 72MHz band we use to the other end of the band is probably in the order of a few milli-meters.