The Cobra 18 looks like a nice radio. As far as fitting, I'd look at the Cobra 19, and the Uniden 510 or 520, they are narrower and might be easier to fit.
I don't like the Cobra 75 all in one style. It makes for a big mic in you hand and without an external speaker the sound is not good.
Firestick's are popular. A 3' or 2' is fine for trail comunication and does not have clearence problems.
Antenna cable should be long enough to reach from antenna to radio with a little slop. There is no "magic" length. Too long is better than too short.