Great stuff, I have been trying to get some replacement pads for my Ax 600's for ages all to no avail...which cartridge pads did you use and were they an exact fit other than chopping the ends off?
Its simple , you need to compare the cadridge brake pads wich are available with the old ones.
The lenght is no issue , its the thickness of the pads,most modern cadridge brake shoes are flatter , iow not as thick as the old 600AX 's.
You could before sliding the cadrige shoes into its groove , first install some backing material (rubber sheet mat cut to the right size) and slide the brake shoe over the backing material.
I don't know if the sell these parts out there , but i used these:
They are quite soft but don't wear that fast , they give excellent braking power!
its from this company:http://www.bbbparts.com/brakeshoes_bbs23ct.php
Only one important thing : you need to be carefull to make the hole needed for the screw to lock the brake.
I used an red hot thick needle .
You could also just cut away a little part of the back,and insert the screw, its no big deal ,the screw only takes care the shoe won't be released from the brake when braking and riding backwards!
Tooling and riding its Jing and Jang