Changing '75 international to clip in pedals and shoes
I would just pull one pedal off each bike and go to a bike shop.  Go with the right pedal as the threads are normal.  I seriously doubt either has odd threads that you cannot find clip pedals.  You would just get compatible (or same) pedals for each bike.   Pick a pair of shoes that walks OK.  A mountain shoe is going to walk better so you should get mountain pedals.  Shimano SPD and Crank Bros Egg Beater are two popular types.

A hard soled shoe (without clips) designed for biking with larger platform pedals is also an option.   They don't bike as well but they walk a whole lot better.  Five Ten is a brand.