Because of the way how tubes are connected. In case you cannot make equal length tubes it would be much better to connect them like he was trying before... Another thing you should look at what turbo you will use because for twin scroll turbo manifold should be different configuration.
Something like this http://www.r3vlimited.com/board/showthread.php?t=153442 but better with equal length tubes.
Thats almost exactly what I said on R3V. At that point, just go with a log manifold and save yourself the complexity! I really don't think there is much to gain by making the 4-2-1 like that because the 4-2 connections are at 90deg. At least angle them to merge them!
Can you find a log manifold that's readily available and clears AC?
Also, OP, you may want to reconsider that bulkhead fitting for the oil return line. I've had those fail, no matter how much locktite or elbow grease you put, it'll eventually leak. No replacement for a welded fitting, and it'll cost you like $20 at a weld shop if you can't TIG aluminum (I sure as hell can't).