19/26 works with stock spurs, which is 63/70. As long as the CB pinions have a difference of 7, they work with stock spurs.
I'm about to install 21/28 in mine... I run smaller/lighter tires, so the extra teeth on the pinion more than make up for the difference.
I ran 19/26 on mine (with stock spurs) and it wasn't enough speed with my setup. I'm guessing my engine will pull the taller gearing just fine though. As long as I stick with smaller/lighter tires. Even so, I'm sure it will give my teflon shoes a workout!