The stock manifold isn't a full split, it would need to be welded to close the split. The idea is to increase the gas speed through the turbine, not slow it down with large T4 castings. Garrett is on the right track with the new split Vband housings imo.
An adapter would work, as long as the split is retained. You don't have much room to work with though. Shame you couldn't get good flowing external gate/s off the spacer...
The BW internal is by far the best design I have seen, but it won't flow as well as a 50mm or two, no.