IIRC the Galaxy changed from 27 x 1-1/4 (or 27 x 1-1/8?) at about the same time as canti brakes were added to the spec....in approximately 1986.
My best bike is a Galaxy from just after that change - and has now been heavily modified to modern triple 9 drive train, STI, nice handbuilt wheels, Brooks Swift Ti Saddle. It's gorgeous to ride.
As Willem says - not worth getting canti bosses brazed on if it doesnt have them.
If it was a 27" wheel version you can put 700c wheels on it and use long drop dual pivot brakes like
these. I have done this on an older 27" wheel Galaxy...it does work.
Either way, you'll need to spread the rear stays to accept a modern wheel.
Measure them. If it's 126mm between the inside faces of the drop-outs you can just spring them apart each time you want to put a 130mm wheel.
If it's 120mm then you'll need to cold-set them. Sheldon has a method. Personally I prefer to push them out using an M10 threaded rod, nuts, and some big washers. That way you can check the symmetry remains OK as you go. Long bit of string from one drop out, around head tube and back to other drop out, pull taught & tie off. - measure from string to seat tube on both sides.
Panniers...my 1987 has the braze on fittings front & back. If yours lacks these you can certainly use P clips at back - not sure if there is a work-round at the front.