Will a YJ hardtop fit a TJ?

It’s not a bolt on as far as I recall. You’ll need to do significant fiberglass work as the windshield height (and possibly angle) is different.

If you can’t find a TJ hardtop, there are aftermarket options out there for a TJ
 
I've seen them on TJs, I think it's a matter of trimming some of the header bar area...

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