Dia Compe Brakes for 1991 Bridgestone RB-T
Advert for Bridgestone RB-T showing brake placement to be a lot closer than standard:



The brakes look like Deore LX cantilevers from the early 1990's (these were available in both silver and black):



I would agree that you have a problem. You may want to put narrow 20mm rims on the bike to mitigate the problem. You could source some early 1990's Shimano/Suntour/Dia-Compe brakes that catered well with touring as well as mountain bikes. Dia-Compe 986's were the first ones to be 'low profile' yet still use the 'brake blocks on stalks' (as opposed to the nut and bolt ones). There were also examples from Shimano and the Suntour badged Dia-Compe models. Pads are also a consideration. When Shimano XTR came out the pads became longer and thinner. In the UK Aztec make replacements for these (with the 'stalks') and they are widely available.

What you may also want to look for are the very early cantilevers that stuck out a long way. Dia Compe 983's are the classic example. There is a problem with these if you are using panniers on the back and if you have anything but a small frame size - they poke out into the pannier bag and get wedged on.

As for getting hold of some of these early cantilevers, try your LBS as they may have some knocking about in the workshop that they may let you have for a small fee.

Reposted from the MTBR forums for future reference, the specifications of how cantilever bosses should be done: