The term "80's raleigh mtb" leads me to hypothesize you could have giant long reach caliper brakes made of stamped steel on what is essentially a fork designed for 27 1/4 or 700c wheels. upgrading to 700 c wheels in this case is easy, you just need some shorter reach calipers. I find old Dia compe brakes are cheap, plentiful and usually work on older low end frames....the only frames i have much experience with. you will probably want to measure the reach or something. my workshop is less than a five minute ride from the LBS with a big ol' selection of cheap used parts so if something isn't working its no big deal to find something that will, as opposed to researching everything down to the finest detail like some kind of nerd. for the record i just did this exact swap on my dump bike department store tandem, so it would, you know, STOP. Steel rims and chinese U- brakes are a scary enough combo on a bicycle built for one.

For the record: if the wheels are decent hook edge aluminum 26 ers and the bike has cantilever brakes(i.e. will not work with bigger rims), why not just get "skinny" road tires?