Ah - I reread you post where you said you still have the old stuff (does that include the housing?)

In that case, I think I'd just pull the spiders out of the new diff and use them to rebuild you old diff. Then you don't have to worry about gear setup and stuff - you can just put it back together with the shims just like it was and the gears should mesh the same.

If the old carrier is too messed up to do that, I'd take the ring gear off the old carrier, put it on the new carrier, and then put that in the old housing. If at all possible, you want to reuse that old housing because the pinion is already in it and set up correctly. If you don't have to fool with the pinion and you're just taking carriers in and out it isn't that big a deal.