[yossarian19]

The washer under the castle nut is fine. As long as the TRE's are the right taper angle (they almost certainly are) and the taper is engaged, no problem shimming the castle nut.
You could just say "hell with the cotter pins" and buy a few deformed-thread locknuts like are used on new cars. NAPA auto parts, at least mine, has a great selection of high quality nuts & bolts. Tractor supply shops will, too. Hardware store hardware is fine for some things but for steering... NAPA or the tractor shop.