Your link just took me back to the spacer page, but from what I remember, “made to order” was pricier. I don’t have my diff apart (there’s a chance I won’t even need the spacers), I’m just trying to have the parts on hand so it does sit like all my other projects. I want to do it all in one day once I get into it.
As to to the issue of “clamp it down and hope for the best” without spacers. Well, I don’t operate that way. I’m one of those guys who hot laps all night long while other folks sit in the pits scratching their heads and cussing. However, I do have concerns about the Z31axles handling 600-700hp even on a rolling start, but that’s a matter for different day. And I don’t have 600hp yet, so it’s getting ahead of myself by a couple of years.