assuming your previous brakes were also cantilevers, you can probably avoid having to cut new housings as the cable routing and stops should be the same. might need a new longer cable though

in general, a short transverse wire will lead to higher mechanical advantage
though more precisely, the angle of the wire defines MA, closer to horizontal is stronger

avid shortys are great brakes, note that they come with multiple brake pad spacers so you can fine tune the arm position; keep this in mind when looking at transverse wire angle vs fender clearance, additionally they should come with 3 available wire lengths
personally, rather than choose between these; I use a transverse yoke hanger and cut a wire to whatever length I need, this gives more control over the angle.....


Yes, you can get 'taller' canti brakes, my tektro cr710s (not to be confused with cr720) just arrived and they are about 1cm taller, thus more fender clearance without steepening transverse wire angle
Velo Orange also has a new canti brake that looks very tall....

i doubt you will need either thought; when I first installed my avid shorty 4s, i actually needed to reduce mechanical advantage since it was too strong to be manageable....