Actually they came in the machine, we have 2 machines and the price was $150 for the replacement knife kit instead of $180. I don't know if they will bolt right on the regular chopper drum or not. I had 2 62's with single hammer style but can remember what the mounts on the drum looked like. Dealer with parts book counld maybe help you.
We changed to fine cut a few years ago while they were still on prototype so I do not remember the cost to upgrade. They do cut much finer than the original. You do replace the entire chopper plus the bottom grate with a new set of stationary knives. I do not remember the year it changed but after 98 and prior to 02 the accelerator rolls were made shorter to accomodate a 2 speed drive pully other wise its a one speed pully and you need to change drive pully to go to slow speed.
I know some guys on their JD straw choppers, to cut finer, they installed longer stationary blades and also installed beveled or sharpened chopper blades which they got at loewens. Might work for youIJ
Paul, I am trying trying make the system less aggressive for baling straw, and that is why I am removing the stationary knives and maybe blunting (rounding) the knives would help when you want to bale the straw.
We have had good luck baling oats straw behind a fine cut chopper set on low speed with the "concave" down and out of the way. The straw isn't full length, but our customers prefer the preprocessing this gives the straw.