Hi PG;
One possibility that no one has mentioned is that the frame could be for ISO590 (26 x 1 3/8) rims. My SR Sierra Sport (mid '80's "mountain bike") was made with canti's and ISO590 rims. For more on my bike:
http://forums.bicycletutor.com/thread-3036.html
I orginally tried fitting a ISO559 (mtb 26") rim, brake pads were way off; measured decided ISO571 (650c) was the way to go. The pads are as far down as they can go on the brakes, work well. 20-20 hindsight, I should have gone with ISO590. ISO584 (650B) is another possibility, but tires are expensive, and not higher pressure.
The brake bosses are too close to the axle for ISO622 (700c) to work, but anything from ISO571 to ISO590 would line up fine.
Suggestion: mount up on arm of a canti or V-brake, measure the distance from the pad center (which is extremely close to the bead seat diameter on most rims) to the axle center. Do this for the min and max for the pad location on the brake arm. Convert to millimeters and multiple by 2 to get the ISO rim bead seat diameter range that will work with the brakes you have on that frame.
Personally, I have found V-brakes to have much stronger braking action that cantis; and much easier to set up. I converted both the SR and T50 from cantis to Avid V-brakes. The T50 was scary with the OEM Shimano cantis; and became confidence inspiring with the Avid SD5 V brakes.