I changed my drive sprocket to a skinnier one when I built my deviate (I needed the extra clearance) gallery384
I used 'drop buster' heavy-duty bmx chain meant for stunting and never saw any stretch...really good chain!
I am not sure what the thread is on the nut though
I broke the nut free by putting a ratchet on it and giving the ratchet a quick smack with a hammer,,,don't laugh, it works !!!
you can then pull the gear with the included gear puller