Look at dual pivot side-pulls or good quality Centerpulls for the brakes , for example,
Velo Orange: http://www.velo-orange.com/grco610cncbr.html

You have a freewheel hub on the back , don't get too ambitious with the bike you have.

Cantilevers are a Brazed on brake fitting, Go forth and buy a new bike ,
with the kind of parts you want.

A cyclo-cross bike frame is made around having cantilever brakes on it ,
Campag group would put that at $3K.