As far as I know, they're identical.
I had to change the head gasket on my D15B Vtec (I have the whole swap) and I used a Z6 gasket.
BUT, I very well COULD be wrong. I know for a fact the D15B has unique pistons (and maybe rods, too) and a few other unique features but I haven't heard anything about the head being unique, I've actually read that the heads on these engines are identical. But I've never messed with a Z6 so I don't know, also, make sure the head is the same OBD as yours. And if it's a 3-stage vtec, then congrats, you just bought a slower engine
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