Pickup spacing will be the same, but you'll have much less space between the end of the fretboard and the neck pickup, which could affect your slap playing, and was the reasoning behind the Modern pickup placemenr. See the difference between a 21 fret JJ, a 24 fret JJ, and a Modern:
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I can certainly see the difference. I would be alright with 21 frets, but I like the idea of having the same fingerboard as my Modern.