The bike is a 1984-85 Bridgestone MB-2. The brakes are Tektro Cr720. Present rims are 1983 or so Ukai. The brake shoes are Koolstop Mountain, the big curved ones with the little tail to whisk away water.

The LBS set up the brakes for the very wide Ukai rims, which I think were used on some of the early 80's Ritchey mountain bikes (the rims take a 2.35 tire). I'm in the process of replacing the Ukai's with Araya RM-20's (19 cm inner diameter), which are quite narrower than the Ukai's. My question: In adjusting the brakes for the narrower rims, do I just have to add spacers or do I need to shorten the straddle wire or raise the straddle wire or completely replace all brake cables?. A lot of "or's" there. Help me out if you can.

Note that I would like to do this myself, so please don't say that I should let the LBS do it. That is the last resort.

I've done the pertinent searches (I think) but could not find the definitive answer.

There are two different sets of washers holding the brake pads/shoes. I would place the wide set on the inside. Then I would set the straddle wire so that there is an approximate 90 degree angle between the wire and the arms when the pads contact the rim.

You want spacers.
As the calipers get closer together, you lose mechanical advantage, so you want them wide apart.

https://sheldonbrown.com/canti-trad.html

Hi,

As far as I understand it canti's can be set up in various ways,
to achieve the same mechanical advantage at the brake lever.

In this case spacers on the pads will give you what you had
before. But perhaps it is a good point to review the actual
advantage that suits you, which is varied by straddle angle.

rgds, sreten.

Thanks for the input. I thought spacers would be the basic solution, and I will take this opportunity to thoroughly study cantilever brake setup, referring to the master Brown-sama as I go along.