I'm one to find ways to work around the instructions when I see a better way (which is a kind way of saying I don't read the instructions....
) but I didn't see any possible way of getting to the removal of the stock lever without loosening the exhaust pipe and basically removing it for all intent and purposes. No way I could see on mine to get to the point of cotter removal and/or master cylinder removal. I figgled, foddled and farted just about every way possible and gave up. I was going to install the floorboard extensions anyway but had contemplated the smaller brake peddle foot pad for a few months and gave that a try. I did devise a cover so when I was grinding the brake lever so the smaller pad would fit, the dust/soot/metal shavings would be confined and not go willy-nilly all over the bike.
I'm glad you are pleased with your choice of the Kuryakyn extended brake pedal....
Remove floorboard.
remove bolts holding rear master cylinder to frame.
remove nut holding lever on but don't try to slide lever off pivot. (be careful of the o rings.
lift the lever and move rearward tilting the master cyl slightly, and from underneath remove cotter pin.
lower lever to remove the pin and little washer.
Reinsert pin from above with long needle nose.