Yes the 38,42x,42m,48x & 54X will fit on any of the X300 or X500
Even if you wanted to down Grade you can put a 42inch deck on the X320,X340,X360 and even any of the X500's But with X500 series the Diff/look pedal Has to be removed
The deere X300 & X500 are bult on the same frame and Have the same deck mounting Points.:thThumbsU
I remember asking my dealer about the 54" when I was in the talking stages of the purchase of my X320. He said it was doable if I wanted it. I ended up going with the 48" though. I thought the 48" would be more maneuverable for me.