Is there anything strange or kinky about removing the mower blades on a JD X500? Like reversed threads on the bolts or are they just tight? Are the bolts metric by any chance? One socket too tight, the next too loose.
IIRC they are 18mm, not SAE, and plenty TIGHT (65 ft lbs) normal thread direction.
Impact... either electric or air (if a big enough compressor behind it) with impact rated socket. HF sells a nice electric impact for under $40 and you'll learn it's great for all kinds of other stuff.
18mm socket on 1/2" breaker bar with a piece of 2x4 wedged in to hold the blade... don't slip
I had to use a REAL BIG Breaker bar.Don't even think of using a simple ratchet.