I was trying to figure out that also. Seems like flow wouldnt be worth a shitit must be the picture angle but it actually looks like the factory oval ports are larger than the tubular race headers? one thing I don't like about those fabricated headers is that the connections are 90 deg straight into the 'log'; i'm surprised that there isn't some sort of curve to encourage flow instead of a 90 degree 'wall' for the hot gasses to bang into.
as an aside the factory cast manifolds do look beefy (which I'm glad to see!) thanks for posting the pics!
PS any time I've ever installed tublular headers on any of my race cars the engine always sounds lots different; usually more valve type noises etc (I think the cast factory manifolds soak alot of that noise up)