Jam a 2x4 or something between the outer blade and deck to keep them from moving while turning the blade nut.
Might want to get PB Blaster or equivalent oil on the nut if they haven't been off for a long time. I always had problems with the center blade nut coming off, was very stubborn!
Nice find, Mike on the Stens cross-over part #
Rattosh51
YT6800