I suggest you mount your cantilever brakes on the fork, then measure the distance from the center of the axle position to the center of the brake pad mounting bolt. This measuremant is called the brake radius. Measure with the pad at both it highest and lowest position. Also put an 20-25mmmm block between the pads, to represent the rim and tie or tape the pads to it so that they are in a natural position and you do not get error due to the arc travel of the pads. Repeat the measurements a couple of times to make sure you are accurate. With these measurements you can pick the wheel size(s) that the frame and your brake choice will handle.

Normally, you do not have to be concerned with 27" vs. 700C as there is only 4mm difference in the brake radius and most cantilevers have sufficient range of pad adjustment to handle this. However, if it a Portage, there is a concern, as these were fitted with 650 wheels. I am not sure which of the 650 versions it was, but usually touring bicycles are 650B.

The brake radius for the various rim sizes are as follows:

27" - 315mm
700C - 311mm
650A - 295mm
650B - 292mm
650C - 286mm

Based on the serial number, I'm wondering if it isn't circa 1988, as opposed to mid-1980s. Can you post a pic? Was the paint job originally two-tone with a contrasting head tube?